2 Colors

http://css3buttongenerator.com/
http://cssgradientbutton.com/

3 Colors

http://freehtml5templates.com/gradientgenerator/gradient-generator.php
http://gradcolor.com/css3-gradient.php?c=3

More colours

www.cssmatic.com/gradient-generator
http://www.css3factory.com/linear-gradients/