How To Have A Good Night Out Without Breaking The Bank
For ideas of fun free or nearly free things to do, have a look at the excellent website London Free List. Or buy a listings mag like Time Out. [The Big Issue]?, a magazine sold by homeless people, also has listings and reviews cheap places to eat - don't be put off by half of the sellers looking like they're loonies, they may be loonies but they're harmless loonies. The Guide (free in the Guardian newspaper on Saturdays) is also very useful.
This is the sort of thing I generally do when I'm broke:
Buy a One-day Travelcard. This is less than a fiver and will let you go anywhere in London until 4am the next morning - useful for the night bus home!
Check out events like the Thames Festival which happens every September and includes loads of free live music and associated events along the South Bank and Bankside - last year there were people making 60ft towers out of sellotape to the accompaniment of music and fireworks - sounds odd but was excellent - and a superb Carnival of Lights which was beautiful. There are random free or nearly free festivals happening all over London pretty much all through the year.
Remember that almost all of the major museums and galleries' usual exhibits are free. As well as being great places to go in and off themselves, they're also good for people-watching. Some churches have free concerts, in particular [St Martin in the Fields]?.
Many bars in London have drinks promotions, particularly in the week. You can often catch good live bands at the same time. Keep your ears open!
If you don't want to travel into Zone 1 then see if there is a comedy club in your local area - many large pubs in the suburbs run a regular night. Again, listings mags are your friend.