This post was originally published in 2012
Some information may be outdated.
Here’s a great set of free commercial use vector buttons from the penguin people here at Animatedbanner.com. The set contains non-resolution dependent .eps and (scalable) .svg files so you can re-size them to any size you want (with no quality loss) and also change the colors and text if you have vector image editing software.
If you don’t have any vector image editing software, you can download Inkscape for free. Inkscape is a free Open Source vector graphics editor, with capabilities similar to Illustrator, CorelDraw, or Xara X, using the W3C standard Scalable Vector Graphics (SVG) file format. If you use Inkscape, be sure to use the .svg format file from your vector button set since it does not currently support .eps format.
There is also a transparent .png file for those who just want use them as they are.