MNPlight aims at getting rid of the need for a computer with iTunes. It runs on your iPhone, and is able to modify the iPhone calendar, address book and iPod databases.
MNPlight can be of particular interest for Linux users who don’t have iTunes, but it can also be used by other users when they want to update their iPhone data when they are away from their iTunes computer.
If you keep using iTunes, you may experiment problems with the Calendar and the Address Book since modifications done on your iPhone won’t be seen by iTunes.