I originally created these buttons to help out amateur XNA developers back when I was experimenting with game development myself. Shortly afterwards a bunch of established studios started to use them too! You can find them in great games like Puzzle Quest and some less than great games like Duke Nukem Forever.
You can use them freely for personal or commercial projects as long as you follow the license. Basically, you have to give me credit if you use them in your game. Full details are inside each Zip.