There's a clever new service from Vyke Mobile* that utilises the fact it costs a fraction of the price to use mobile internet than it does to text. If you've a phone which has web or WAP access, which is most phones bought in the last three or four years, you can take advantage.
Just download the free software, and then text from your mobile to other UK or worldwide mobiles for a fraction of a penny. The system feels virtually identical to normal texting (you can even use your phone's address book) but is actually sending the texts via your mobile's internet service. Better still the message comes from your own mobile phone, so mates can reply easily.
Saves me about £5 a week texting my parents who live in Bulgaria.