Tracks YuPack, LetterPack, Registered and CoD mail, Seino, Fukutsu, Amazon TMG. FedEx Express, international EMS and almost everything you may track from their web sites -- you don't even have to know the package type, just enter the number and the program will automatically check and decide where and how to track it.
Tracking numbers can be entered manually or using barcode scanner (ZXing "Barcode Scanner" from the Android Market), and are immediately checked for validity, thus eliminating 99% of typo errors. Barcode scanning does actually work (I've tried a few other applications, they also claim barcode support but fail to scan most of Yuupack labels).
Depending on your locale/language settings, tracking info is downloaded from web sites in English or Japanese, except for Sagawa Takkyubin and Fukutsu, which do not provide tracking info in English.