We've written about Jajah here before. It's essentially a VoIP calling solution you initiate by plugging in two phone numbers into a web page. If both numbers are Jajah subscribers, the call is free. If not, you pay a small (but slightly higher than Skype or calling card) fee.
Now Jajah's taken their mobile web page and made it PSP compatible. Now, this doesn't quite mean that you can make calls with your PSP. It just means you can use your PSP with Wi-Fi and initiate a Jajah call to go to your cellphone. Useful if you're using a really old phone that doesn't have at least a WAP browser.