Let’s not forget that security software serves the purpose of keeping your computer secure, but there’s a point in that only if you use your computer for something else! 
So… Programs outside the security field that I use and find great:
ACDSee 3.1. I’ve heard that the latest versions of ACDSee have become bloated, but this little baby that came included with my scanner back at the time displays images faster that I could think about it. Also it includes some great basic editing/photoenhancing features.
If you’re looking for something free in the same efficient lightweight line, I’ve heard good things about FastStone’s software but I can’t vouch for it myself.
Foxit Reader. This one’s freeware. I can’t believe there are still so many people out there waiting those annoying seconds before a simple PDF is opened. Even before I tried any alternative, I already thought that Adobe Acrobat was unreasonably slow. Well Foxit reader is unreasonably fast! ;D And last time I checked (when I used Adobe’s freebie for the last time) it renders better too.
VideoLAN VLC media player. As Soya says it’s great, since Windows Media Player doesn’t include support for DVD 88) and other formats for free, this is the best alternative I know. I actually planned to replace it WMP as my default player for any format, but discovered that it’s not 100 per cent bug-free so I fell back to WMP for its supported formats and use VLC for DVDs. 
OpenOffice. Fully featured, does the job great. Can’t really compare its features with the latest versions of MS Office since the last I used extensively was 97, and thanks to OO I’m not missing MS’s solutions. Also I haven’t noticed any bugs, always work with OO’s native formats when possible. It’s not optimized for speed or resources, but neither is MSO, is it?
Opera. Of course!
Simply the best Internet experience for me. Awesomely fast, fully featured, bug-free, efficient…
JkDefrag. Sooo much better and faster than Windows’ defragger.
WinRAR. Welll it’s the compressor I use and it works great, and supports lots of formats with no glitches whatsoever. Tried 7-Zip and I guess it’s great 99 per cent of the time, but it consistently crashed when trying to open one certain big ACE archive; WinRAR did the job so I dispelled my doubts and paid the very reasonable fee for a life-long licence.