Looking for a good texting applications

I'm not sure about making it seem like your phone sent the text, but you can send texts from iChat. Just send the message to the phone number with +1 added to the beginning. For example, if your friend's phone number is 617-xxx-xxxx, type +1617xxxxxxx into the "To" field of an IM, and the message will be sent to their phone in the form of a text. It'll say that it's coming from your screenname, though. I do this all the time when I'm at my computer, so what I ended up doing was adding the +1617xxxxxxx number into my close friends' address book entries as a second screen name. Now when I'm in iChat, I can just right-click on their name and choose where to send the message to; either their AIM screenname or their phone. But, there are no ways to type a text on your computer and have it be sent by your phone number...as far as I know!

EDIT: Ooh, just noticed that you're in London. I'm only familiar with this method on American phone numbers, and am not sure how it would work outside the country...