Version b5b fixes an issue with using custom ramdisks on iPhone3G and iPod2G, and version b5c prevents redsn0w from crashing due to the ever-growing ramdisk size :).
TIP: If auto-detection fails and redsn0w tells you no identifying data was found, you can always pre-select the appropriate 5.0.1 IPSW using "Extras->Select IPSW".
You can download RedSn0w from here.
You can find instructions on how to jailbreak your device here.
Read More [via Sidney]