I'm a doctor, not a mechanic
UClasses's main functionality is being a stand-alone Unreal classes browser with full-text search capabilities (something UnrealEd's built-in actor browser lacks). In addition, it provides functionality like UnrealEd for adding new classes, compiling packages and so on.
Download and information: UnrealTower (Warning: Flash AND HTML site ahead!)
Created by Meltdown.
- Other applications
TaoPaiPai : At the date of october two-oh-six, unrealTower's website is down. Uclasses can still be downloaded at the following adress  although I don't know how useful it is nowadays compared to the other applications (do any body still mod for UT ?)...