For an extensive list of Web2.0 software, go to IT Redux (via the DEN Blog).
Sudeep Bansal has a good list of Essential Freeware for the PC User.
Another comprehensive list of freeware can be found at 121 Space.
Steve Froemming has a list of Academic Freeware.
And for a short list of freeware for Mac OS X, go to "Open Source Mac".
