Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
One of my favorite open-source applications is Samba. For me, it, and not Apache, is the Linux "killer" server app that I use all the time.
That's because at my home office -- with its score of PCs and half-dozen servers -- I run a mix of servers running Linux, Windows 2000, and Windows Server 2003. I also consult from time to time, and most of my work is with similar networks or Windows networks on their way to becoming Linux server-based LANs.
For these situations, Samba is invaluable.