I found a few:
http://github.com/ssoroka/ansi/tree/master
Examples:
puts ANSI.color(:red) { "hello there" }
puts ANSI.color(:green) + "Everything is green now" + ANSI.no_color
http://flori.github.com/term-ansicolor/
Examples:
print red, bold, "red bold", reset, "\n"
print red(bold("red bold")), "\n"
print red { bold { "red bold" } }, "\n"
http://github.com/sickill/rainbow
Example:
puts "this is red".foreground(:red) + " and " + "this on yellow bg".background(:yellow) + " and " + "even bright underlined!".underline.bright
If you are on Windows you may need to do a "gem install win32console" to enable support for colors.
Also the article Colorizing console Ruby-script output is useful if you need to create your own gem. It explains how to add ANSI coloring to strings. You can use this knowledge to wrap it in some class that extends string or something.