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Some wags (no one in an official position, however) are proposing renumbering the Capital Beltway to Interstate 666, to signify the depravity (political, cultural, fiscal, whatever) thriving inside its confines. It intersects I-66 and is a beltway, so the I-666 designation would be Aashtoically permissible.
I-495 Delaware (link)
11.47 miles ; serves Wilmington. In the past (1970s) this was part of I-95, while the parallel section of I-95 was numbered I-895. I-495 was completed in 1978. 
Will I-495 be absorbed by I-95?
On January 19, 1999, the Wilmington News-Journal reported that the American Association of State Highway and Transportation Officials (AASHTO) are considering redesignating I-495 as part of I-95, to encourage through traffic to prefer the wider, less-traveled, higher speed limit outer route (today's 495) over the narrow downtown route (today's 95).
Dan Mengel adds: "...it makes sense. The original 95 through Wilmington is 4 lanes, part of which is on a viaduct and part blasted through a rock cut and the center of the city (four churches were demolished in its creation). I-495 is 6 lanes with shoulders. Recently both Delaware and Pennsylvania have put "Local Traffic" markers on the I-95 signs and "Thru Traffic" markers on the I-495 signs at the interchanges." 
If the renumbering goes through, the current downtown I-95 route would be redesignated as Business Route I-95. 
You might ask why the roads need renumbering when the "Local" and "Thru Traffic" signs were already erected for the same reason. It turns out motorists appear to prefer following 2-digit routes. During recent repaving along I-95, traffic was successfully rerouted to 495... but when the paving was complete, much of the traffic came back.  A continuous "95" numbering along the bypass would help drivers along.
In that case, you might ask what's holding up the renumbering. Answer: politically influential businessman Henry Topel, according to the local press. He's a former Visitors Center board member, a former four-term Democratic Party Chairman... and owner of a hotel whose business he fears would suffer if I-95 was moved. "He singlehandedly held back Gov. Carper and state senators from making the change," states a May 2002 News-Journal editorial. 
In late 2000, I-265 in Nashville was absorbed into I-65, for similar reasons, with less controversy.
I-495 Maryland; Virginia (link)
64 miles (approximately); this loop around Washington, D. C. is the Beltway in "Beltway politics." The first section opened in 1961; the last, on August 17, 1964. Near the New Hampshire Avenue interchange, Maryland Governor J. Millard Tawes cut a ribbon officially opening the Beltway. Remarks from officials included: 
Beltway Route Numbering
In 1958, Virginia requested the number I-68 for the Capital Beltway (proximity to I-66 and I-70S). AASHO denied this, and the number was eventually set to I-495. .
For 11 years, the entire Beltway was numbered I-495 only; I-95 was planned to pass straight through, close to downtown Washington. In 1975, after this plan was cancelled, I-95 was routed along the 32-mile eastern half of the beltway, where the I-495 designation was removed. 
However, a beltway with two different route numbers (495 and 95) didn't suit motorists well. On June 9, 1991, AASHTO approved restoring the I-495 designation to the entire beltway. . The eastern half of I-495 now overlaps with I-95.
In the 2002 FHWA highway log, however, there is no overlap: the eastern half of the beltway is I-95 only. The I-495 signs there are for motorist convenience, not an FHWA designation. I-495 officially has 15.28 miles in Virginia, 16.13 miles in Maryland, and no mileage in the District of Columbia. 
Springfield Interchange Makeover
The "mixing bowl" interchange at I-395 and I-95 is undergoing a $320M reconstruction, which started in 1999 and should finish around 2007. (The entire beltway originally cost about $900M in 2001 dollars.)
The five-level interchange will be 100 feet high and 14 to 25 lanes wide. Longer ramps will let traffic exit well in advance of the interchange, eliminating the weaving and crossing that goes on now. 
X is for Extreme
The Cabin John Parkway, a 1.5-mile stretch of road leading from I-495 exit 40 in Montgomery County, Maryland, is officially I-495X, according to the Maryland Department of Transportation.  It's not signed that way.
Maryland also maintains a secret I-270Y designation for the I-270 spur.
I-495 Massachusetts (link)
121.56 miles . For a long time I-495 was the longest 3di in the world. Now that honor goes to Pennsylvania's I-476, the interstate equivalent of a short man on stilts: it was extended along a branch (formerly PA 9) of the Pennsylvania Turnpike.
From I-95 near the New Hampshire state line, I-495 circles Boston at about a 30-mile radius, ending (with I-195) at Cape Cod's front door in Wareham.
The portion between MA 2 and US 3 dates back to 1960. The newest section, between I-95 and MA 24, opened in 1982.
"I think a Bostonian would rather go north or south than east to west. There is a sense that anything west of Route 495 needs a passport and currency control."
John Mullin, director of the Center for Economic Development at UMass 
I-495 (not signed as interstate) Maine (link)
About 2 miles long, I-495 is the unsigned Falmouth Spur connecting the Maine Turnpike to I-295 north of Portland. This designation was part of a January 2004 changeover where route numbers and exit numbers were changed to make area highways easier to navigate. 
The Maine Department of Transportation (MDOT) decided to leave the Spur without I-495 signs, to avoid potential confusion with nearby I-95 and I-295. 
I-495: originally a 50-mile stretch of Turnpike
Before the January 2004 changes, I-495 was the designation for a 50.47-mile stretch  of the Maine Turnpike, leading northerly from the Falmouth Spur to the I-95 junction at Gardiner. This had been in place starting around 1988. Before that time, this section of the Turnpike had no number.
MDOT's effort to unclutter, clarify Turnpike numbers, exits
In 1999, state and turnpike officials began discussing how to clear up motorist and tourist confusion over Maine Turnpike route and exit numbering. 
For historical reasons, exit numbers were sequential instead of milepost-based, which is generally regarded as more useful. (There's some discussion at my Exits in Connecticut page.) Even worse, exit numbering along I-95 northbound restarted at York, several miles into Maine, where the Turnpike began.
Route numbers didn't follow physical highways, either. I-95 followed part of the Turnpike north of the Portland I-295 split, but then crossed over the Falmouth Spur to take over the Portland-Brunswick route that I-295 had started. Meanwhile, the Turnpike north of the Spur suddenly became I-495.
The Maine DOT and the Maine Turnpike Authority decided to establish the following in early 2004:
These are sweeping changes, but the outcome is a much more orderly, rational system. New route signs were installed staring January 5, 2004,  and new exit signs will be erected in May 2004.  See also:
I-495 New York (link)
66.38 miles ; the Long Island Expressway ("LIE") starts at I-278 in Queens and ends at NY 25 in Riverhead.
The original I-495 route defined in Oct. 1958 extended from the Queens-Midtown tunnel to the Clearview Expwy (I-295).
In anticipation of a proposed Mid-Manhattan Expressway near 34th St, the designation was extended westward via the Lincoln Tunnel to the New Jersey Turnpike. However, the expressway was cancelled, and in 1989 the section of I-495 in New Jersey was demoted to state route 495.
East of I-295, the LIE was called NY 24, which was changed to NY 495 in 1962. Despite the number continuity, Long Island proper still had no interstate service, and lobbied hard for extending I-495 over the NY 495 section. The FHWA approved this in 1983. 
If a proposed Long Island Crossing bridge had been built to Connecticut or Rhode Island, I-495 would have connected to I-95 again on the mainland, making the even first digit (4) more strictly appropriate.
The first one-mile segment from the tunnel, the original Queens-Midtown highway, was a viaduct built in 1940. It was finished in Queens by 1955. It was completed to Riverhead (Exit 73, County Road 58) on June 29, 1972.
See also (west to east):
I-495 (numbered as another interstate) Pennsylvania (link)
This was an old numbering for the Blue Route (now I-476). It was changed to I-480 on Nov. 12, 1958,  and to I-476 in 1963.