I generally use Apache 2.0. I feel it's a pretty open one allowing you to use it for personal or commercial, while not necessarily requiring recontribution. The recontribution clause (a la GPL) is what stops most companies from using GPL packages. I fully believe it is very important for people to recontribute back to the community, but I also realize that we live in a real world and some code is proprietary and should not be exposed outside of the business, so forcing someone to just doesn't seem right