We have always chosen to release all our themes and plugins under the GPL v 2.0 + license. This license is the same one that governs WordPress, our favourite software. We work exclusively on WordPress so being 100% GPL is a no-brainer. There have been many questions raised on where GPL can be enforced and where it cannot be and some have even questioned if GPL is really a viable license.
At rtCamp, we prefer to not look into technical yet sometimes abstract discussions but rather focus on what are benefits of something we use.
For starters lets look at the four rules are entailed in the GPL license.