Play a Stream is a free and easy to use Applescript that will play an MP3 Stream chosen from a list stored in a file.
This can be stored in the iTunes Scripts folder or run from the Script Menu in OS X (and other Script Menus as well.)
The file "urls.txt" can be edited to add or delete genres and sites. It should be a return-delimited list of two items on each line delimited by a tab.
The genre should be one word and the site should be a fully qualified URL. Shoutcast and Live365.com are two places to find many other MP3 streams.