Currently you can download LimeRa1n for windows only. Mac and Linux versions are under development and are not available at present.
Download limera1n from HERE.
Changelog:
• BETA 1 – First release
• BETA 2 – Fixed kernel patching magic
• BETA 3 – Fixed new bootrom 3GS
• BETA 4 – Uninstall fixed, respring fixedish