What is a decent quality free video editing software?

I have Windows Movie Maker and use it all the time, but I want to do more advanced stuff like placing captions in specific places on the screen and hooking together multiple videos onto one screen. On my music covers, for example, I want to have myself playing lead guitar in one corner, rhythm guitar in another, and then b in the last corner. Can you recommend some decent freeware and tell me where I can find it?