What's on in London?

Where is the best place to find out what is going on in London over any particular time period?

I'm planning a visit up to see a friend and trying to plan what we can do whilst I'm up. There aren't really any West End shows that I'm desperate to see, so just wondering where I can find out wider entertainment listings.

There are so many venues for entertainment, I just wondered what the best resource was for finding out what is on across Theatre, Music, Comedy, Magic, etc rather than venue specific listings.

Looking at either the last weekend in March, or the first in April.