In addition to what Sylv provided, you can check out the console-only Shoutcast tool for 10.3 and higher if you're comfortable in the Terminal:
http://yp.shoutcast.com/downloads/sc1-9-8/sc_serv_1.9.8_MacOSX.tar.gz
You should be aware that your own outgoing bandwidth is unlikely to support more than 2-5 (at most) listeners at a time, so you may want to check out services that offer the ability to help you support a larger audience, such as Live365:
Live365 Internet Radio Network - Listen to Free Music, Online Radio
You may also want to consider another option: podcasting rather than broadcasting. Broadcasting gets expensive real quick, requires live service (and generally requires 24/7 operation if you want to seriously build an audience, lots of licensing (if you're using any music that's not your own) and tons of bandwidth.
Podcasting lets you create programs at your leisure and put them on iTunes whenever you want, for free (well you have to host the actual podcast files but you can do that on a wide variety of hosting sites, shouldn't cost much if anything). Worldwide audience, easy as pie using built-in tools like Garageband or whatever you like, can be video or audio or enhanced audio (audio with still images).
You still have to have a license to use copyrighted material that you don't own, but its still WAY cheaper than trying to broadcast live in most situations.