Ayende's DivanDB (also, is esent the most widely used database engine?)
Esent is an important part of the Windows Server story, providing storage for the Active Directory and Exchange. Since Windows XP it has been used by several Windows services (run taskmgr -m esent.dll to see what processes are using esent on your machine). The presence of several esent databases on every Windows XP/Vista machine means that esent is probably the most widely used database engine out there. Although the API has been available for years, I'm not aware of that many non-Microsoft project that are using it. It is nice to see other peope considering it for their data storage.