Kik. Works on every major smartphone platform, including BlackBerry. Faster than BBM and includes BBMs feature of showing whether the recipient has seen the message, and it's a free download.
GroupMe works pretty well. The iOS app is good, while the WP app needs a bit of work. But the good thing is that GroupMe can send and receive messages through regular text, plus you can message from their web site. You will need a phone number to receive a pin code, but just enter it on the iPod touch and your good to go.