Server software
Zentyal is an open source alternative to Microsoft's discontinued Small Business Server (SBS) that provides Active Directory and Microsoft Exchange support, among other things
Software tools
General software toolkits
Helge's Switchblade is a "portable USB toolkit" that can be used with Windows XP, Vista and 7 for diagnosing and repairing a variety of problems
Windows product key tools
ProduKey is a tool that can be used to read the product keys of installed programs
File management
Data recovery tools
File duplicate removal tools
WinMerge is an open source differencing and mergint tool
DoubleKiller is a free program that searches single or multiple directories for duplicate files
easyDuplicateFinder is a free tool for finding and removing duplicate files
dupeGuru Music Edition is a tool for finding duplicate files, and also includes extensions useful in dealing with audio files
Duplicate images finder is a tool for finding duplicate files, and also includes extensions useful in dealing with image files