First Deal for me, but I like! ;)
Lebara Mobile offers now free mobile internet until 30. June 2010. You don't need a Topup or credit .The sim is on their webpage for free (delivered)!
(I am not sure if it works when you buy the sim card without any credit, maybe just topup a few quid, don't know if there is a minimum).
Most important: It is free only up to 10mb per day, then 15p per mb thereafter.
A great offer for existing customers but also for those, who just want to check emails, facebook and google, where ever they are. Remember Skype and messengers!