Amazing open-source software
Miro
Video Player and Bittorrent Client. Supports .mkv and .avi without any extra codec pack installations, haven't seen any tearing or downsides to it so far, shows subs perfectly as well. Also has support for Video RSS feeds. Relies on open-source codecs for playback as well, including a lot of the codecs found in CCCP.
Miro Video Converter
Convert almost any video to MP4, Ogg Theora, or Apple formats.
SkipScreen
A Firefox extension that skips unnecessary clicks and wait times on Rapidshare, Megaupload, Mediafire, zShare, and more.
Thunderbird
E-Mail client from the developers of Firefox, Mozilla.
RSSOwl
Self-explanatory.
Cabos
Gnutella network P2P client. - Though I don't recommend using the Gnutella network to download anything, since Limewire is shutting down, this might appease some people looking for a better client.
AbiWord
Leaner and faster than OpenOffice. Word-editing program only, this is not a giant suite like OpenOffice.
Juice
The best podcasting client out there
Paint.NET
Lighter and faster than GIMP, with many of the same functions. Requires .NET Framework 3.5 SP1 or higher.
Inkscape
Vector Graphics application. Definitely one of the better ones.
Filezilla
By far the best FTP-Client.
Peazip
Fantastic archiving utility to extract and pack archives of an extensive amount of formats. PeaZip also features password protecting archives, keyfile support, system integration, a lovely interface, and much more. Includes .7zip/.kgb compression as well.