Its very cool. You can use all your IM clients in one place. Though there are some issues with this software but I hope it would stabilize in upcoming versions. It saves your money you spend sending SMS.
For more information please visit the following link
http://www.agilemobile.com/iphone.ht
Needless to say that you need to first jail break your iPhone to install softwares unless you are ready to pay and get it from iTunes. I have been searching and trying softwares for quite some time now and I thought it would be worthwhile to share my experiences with you.
Below is the list of software which I think can add a great value to your iphone ofcoure it is quite dependent on person and his likings and profile etc but you can always choose what you find interesting and ignore what you would hate.
1. SummerBoard
2. GRE Vocabulary
3. Call History Patch
4. MxTube
5. SendPics
6. Wallpaper
7. AgileMessenger
8. PhotoDial
9. weDict
10. Docs
