Highgate Cemetery

Absolutely huge historic cemetery.  Divided into two parts; this is the West side, which seems more historic, with larger mausoleums and architectural features like this.  The East side is more packed full of graves, and has a few more modern gravestones (usually done very creatively, not just standard that you'd expect to see in a typical cemetery).
Getting a plot here costs about £20k, it's the most expensive cemetery in the UK.
I spent about 4 hours walking round and didn't cover it all. 
Graves I saw include, in no particular order:
Alexander Litvinenko, George Michael, Ian Holm, Lucian Freud, Karl Marx, Patrick Caulfield, Jeremy Beadle, Malcolm McLaren, Douglas Adams, George Eliot, Paul Foot.  Many of the most interesting graves weren't those of the most famous residents.