I haven't found any I'm particularly impressed by.
I used to use
Spamato (
www.spamato.net ), which had an impressive list of features, was open source and multi-platform. It did what it claimed it could, but it was also a fairly complex app to get your head around. After a while, I decided it was more trouble than it was worth and stopped using it.
One problem with Spamato is that development seems to have slowed down (big time!) over the last year, so it might not be worth investing the time in getting to know it. Especially now that Windows 7 is looming and may not be supported.
Since I don't receive huge amounts of e-mail, I usually just use a simple mail notifier. I'm currently using
Poppy for Windows (
http://www.jsonline.nl/Content/Poppy/Poppy.htm ) which lets me preview and delete messages before they hit my mail client.
About.com has a list and mini-review of the top 14 antispam apps:
http://email.about.com/od/windowsspamfightingtools/tp/free_spam.htm
Gizmo's has some write-ups as well:
http://www.techsupportalert.com/best-free-spam-filter-for-experienced-users.htm
My GF likes
POPFile (
http://getpopfile.org/wiki ) which got favorable comments at both the above links.
