If you want a wild night in London off the beaten track, with an edge of danger (without any real risk, CCTV is everywhere on the main drags) Brixton is the place for you. You come out of the tube station to be assaulted by the sounds and smells of a truly unique community: this is an area that became host to a very large number of Afro-Caribbean people in the '50s and '60s and still has a sizeable Irish population. It's also a good place to find the most evangelical street preachers you'll ever see, often standing right next to a mixture of destitute and lost souls. Don't let that put you off though, there's a huge sense of togetherness here and a big vibe. It's like the Harlem of London. See here for more: www.urban75.org/brixton/clubs/index.html.
The biggest clubs here are the "Fridge" and "Mass", regularly attracting big crowds and hosting a wide range of nights to suit every taste. Both have a bar for warming up, the Fridge Bar (which serves a mean Absinthe), and the Bug Bar under Mass, which also serves respectable modern snack-food. If you life Drum and Bass, the George IV up Brixton hill is as authentic as you can get. Going up Brixton Hill, turn right just after the prison and you can find a real Windmill with a pub under it: perfect for winding down the next day. There is also a massive park nearby (Brockwell park, Tulse Hill) that does hot food and ice-cream in Summer. For chilling out, Coldharbour Lane has a couple of nice bars, one where I met my wife 7 years ago (the Dogstar). During the daytime this and the surrounding streets and alleys hosts an incredible food market, plus stalls with costume jewellery, wigs, clothes, and the rest. You have to go just to see the colours, feel the sound and smell, well, the smells. I'd advise you to be reasonably careful after dark though - just keep to the central area and don't go wandering into any of the housing estates. Otherwise a great place to hang out (and live, I was there for 5 years).