Great thread idea Boris.
Opera (
http://www.opera.com) : Simply the best browser for me. I've been a fan for 10 years now, they already had tabbed browsing at that time and countless other innovative ideas. I could expand on this subject, but to make it short I love the way it behaves and all the power-user-friendly options and tools available (though I also like Firefox).
Trillian (
http://www.ceruleanstudios.com/) : already mentioned in the thread
Mathematica : great for quick and easy symbolic calculations. I mostly use my own programs for numeric simulations (I must admit I never really bothered to learn Matlab but I've noticed it's powerful at the hands of others).
PCalc (
http://www.analogx.com/contents/down...gram/pcalc.htm) : a good calculator, I use it all the time.
UltraEdit (
http://www.ultraedit.com/) : powerful editor, mostly for programming. I use an older version than the most recent one.
SysInternals (
http://www.sysinternals.com/) : Advanced system tools by Mark Russinovich, recently bought by Microsoft.
CommView (
http://www.tamos.com/) : Network sniffer. I use it for network programming or when I need to know what data a program is sending/receiving from my computer.
Proxomitron (
http://www.proxomitron.info/) : Local http proxy. For when you want to easily change some of the page elements you receive (for ad blocking purposes for example) or http headers you receive or send (for deception/security circumvention purposes maybe). Very powerful. Unmaintained for several years now
I must have forgotten several other excellent programs but, not having used them recently, I cannot think of any more now.