First off, I want to let you know that I have switched from a Palm Treo to a Windows Mobile Treo. I love WinMob! I can't believe how powerful the OS is. I can do many things with my WinMob Treo.
What is fring? It is a free application that allows me to skype while using my data plan and not my minutes. With fring you can also setup your IM from Messenger, Gtalk and ICQ. So not only can you call you skype contacts via your data plan, you can IM with your friends as well.
Another thing you can do with fring is twitter. Yes, you can get tweets to your Treo, or any other WinMob deivce. I think that twitter is a little annoying on fring so I don't have it going. Well, if you wanna give this app a try you can click here for fring.