O2 Free text messaging
O2 lets you send free sms if you register as a user of their site.
You need a mobile phone to register with O2, but it does not need to be on their network. You get 10 free sms, and all subsequent sms are charged at 5p per text. Service has been around for some time.
Free SMS received very quickly as you might expect with O2, and you do get the option of being able to reply to messages too. Send messages up to 328 characters too. Worthy registering, I think.
And of course the O2 website has lots of fun and games etc. including
Pros: Free SMS site, up to 10 free sms, and other stuff too
Cons: Registration required. Only 10 free sms (5p/txt afterwards)