FILExt - The File Extension Source:
Great site to help find out what an unknown file is, based on it's extension. Provides detailed information like MIME types, applications that create it, links to resources etc.
Freeware (and Open source) program to visually report on the differences between folder contents and for comparing individual files.
Doesn't do side-by-side folder compares, and limited to mainly text based file compares (it has some plugins for other formats). Better commercial programs include Beyond Compare and Araxis Merge (which has TRIPLE file and folder comparisons!!!!).
Unison File Synchronizer:
Interesting program to synchronise two seperate collections of files. More advanced than a directory compare program (like WinMerge), it tracks what additions/changes/deletions have occured in both locations and sends the changes to sync back up. Works a bit like rsync, but more orientated for the user and not an Administrator.