The Chronic Dev team released the Mac version of their jailbreak tool Greenpois0n RC earlier and now you have the Windows version of the same. With this tool you can untether jailbreak your iDevices including iPhone 4, iPhone 3GS, iPod Touch 2G (all bootroms), iPod touch 4G, iPod touch 3G (all bootroms) and iPad 1G running on iOS 4.2.1. Dev team is promising AppleTV 2 support in RC6.
This GreenPois0n RC5 jailbreak tool supports animated custom boot logos which is pretty new feature that lets you custom set an awesome logo . Also with GreenPois0n RC5 you can jailbreak the aforementioned iDevices without using SHSH blobs for iOS 4.2b3. Both Mac and Windows version has been updated to GreenPois0n RC5 Beta 2 that fixes many bugs and issues that people were having earlier.
For jailbreaking iPhone and iPod Touch running iOS 4.2.1 check this guide on Mac.