I just recently discovered the Mozilla internet browser which is an open source project & free! I love it. Much better than Internet Explorer with more functionalty built in.
What other great software (open source or not) have you discovered?
Popfile automatic email classification is great, it is also open-source.
Free Surfer MkII, does a great job at killing popups.
Ad-Aware is nice to clean out spyware.
This program called Sharescan2 to search for shared files on my colleges computer network. So much faster than using the Microsoft Network connections for browsing the shared folders of people’s computers.
Zoom Player. Other than not playing CDs, it’s ALL I use for audio and video. Of course, I’ve gotten the Divx codec too, but it accesses it. OK, I guess I use Quicktime when I have to, but that’s pretty rare. One of the things I love about Zoom is that you can set the scroll wheel to skip ahead and back 5 seconds, so if you missed something just then, it’s not this major procedure to get back to it like it is with WMP or even Winamp.
I use Netscape 7, which I hear is pretty much Mozilla. I LOVE tabbed browsing, and Netscape now beats Opera because I can open links in tabbed windows behind the current one by clicking with the mouse wheel, and it works better with Javascript. I HATE IE.
Ok, except that Netscape needs one more thing. If it crashes, it will not recall the pages that were open when it crashed. Opera will. That is a killer feature.
NoteTab Light is a great Notpad substitute. Arachophilia is a nice HTML editor. WinAmp 2.8x is the standard MP3 player. Spybot Search and Destroy is also a great spyware killer. ZoneAlarm keeps your computer network connection safe.
OpenOffice.org - free & open source complete office suite.
Phoenix - browser based on Mozilla, but without email, composer, etc. Only 6 mb download. Blocks pop-up ads and has tabbed browsing.
RagTime Solo - professional business publishing software. Commercial license is $495, but it is free for personal use.
Microsoft Powertoys - various utilities for Windows
I’ve Netscape/Mozilla - IE - Opera all installed. Yet Opera is my default browser. Except for some pages with java.
Sasami2k is a my preferred media player; next is Zoom.
Openoffice is decent enough.
I work in computer graphics, so the following software are my picks
Photoshop
Painter
3dsmax
XP 
If you try download.com & search for a software, you can see how many people downloaded it & then you know what’s hot I see Yahoo Messager has millions of downloads.