Jailbreakme is a browser based jailbreak for the iPhone. It was released as an easy to use way for people to jailbreak their iPhone and user applications and services that Apple doesn't officially approve of.
However, reports are out that Jailbreakme is actually causing a lot of headaches. Some users claim that their iPhone's are crashing more frequently. Others claim that certain software like Skype stops working all together.
In July, it became legal again to jailbreak an iPhone or iPod or iPad. However, Apple still has the ability, with updates, to undue programs like Jailbreakme. Also, phones that are jailbroken have their warranty voided.