Get Into London Theatre (GILT) scheme, run by The Society of London Theatre, has been going for a few years now. I've been several times and always found it well worth it. As a dedicated consumer, I always like a bargain!
You can get tickets at discounted prices for loads of West End shows - not just musicals but also plays, opera, ballet and other entertainment. But it only runs during the quiet season, this year from 10 January to 17 March. So if your idea of London nightlife is a good show, and you want to take advantage of the special prices before this year's offer period ends, best book quick - only a few weeks left to run! Check out the listings of available shows. You can book on the Net.
They've also teamed up with restaurants, hotels and travel and parking companies to offer special deals from them.