Mornington Crescent station was built in 1900, and originally called 'Seymour Street'. By 1907 it was given its present name, and, apart from its use as the title of a popular game (one that apparently originates from Taxi
drivers, and predates 'I'm Sorry I Haven't a Clue
'), it was completely unremarkable until 1992, when it was closed for extensive repairs to its elevators.
A complete refurbishment followed, and due to rising costs it looked likely that the station would never open again. During this time the name remained on the London Underground map, but it was covered with a mysterious red 'X'. The station was still visible - dusty and deserted - but the trains powered past it without slowing. After six years of inactivity, funding was forthcoming, and the station was repaired in late 1997, opening in March 1998. It's now a grade 2 listed building.
It's within easy walking distance of Camden, and Camden Town station. Due to overcrowding, the entrances to Camden Town are often closed, only allowing passengers to exit - in which case, the guards will helpfully direct you to Mornington Crescent.
Why is it called 'Mornington Crescent'? The station itself is directly opposite a road which is crescent-shaped. And called 'Mornington Crescent'. So now you know.
In 2002 a plaque was unveiled in the station, commemorating Willie Rushton, one of the stars of 'I'm Sorry I Haven't a Clue' and also of Private Eye magazine, amongst many other things. Thanks to Withnail for that.