You mean Chroma Key? (e.g. you are sitting with a green/blue backdrop behind you, and you want to animate it? This is the way weather forcasters do it)
You can use t@b ZS4 for that, and it's free:
http://www.zs4.net/downloads
Commercial software includes Sony Vegas Movie Studio (imo this is the best), Pinnacle Studio, and Adobe Premier.
If your just looking for backgrounds though, there are some good free textures and backdrops at BurningWell, they are free even for commercial use. Link: http://burningwell.org/