I'm the email admin for a company that has been using the commerical version of CanIt for more than 2 years now. It is the best Spam filter I've ever found, probably because it builds on Spamassasin, MimeDefang and Sendmail. I found the install to be very easy and the documentation of high quality (as you would expect from a commercial program). Now that it's free for under 50 users, every small business and private user that runs their own mail server really should use it.
An open, cross-platform journaling program.
A scientific plotting package.