The iPhone Dev Team has released RedSn0w v0.9.10 b5c for Windows and Mac users which includes fixes related to iPhone 3GS, 3G jailbreak issues and prevents redsn0w from crashing due to the ever-growing ramdisk size on iPhone 3G and iPod 2G.
Download Redsn0w 0.9.10b5c for OS X
Download Redsn0w 0.9.10b5c for Windows
See also:
How to Do iOS Jailbreak with Redsn0w
How to Do Jailbreak of iOS 5.0.1 with Corona
How to Jailbreak iOS 5 and iOS 5.0.1 with Ac1dSn0w