I used McAfee SpamKiller (or was it SpamCop?) for a short time.
I have a pop3 account, and when you run this software, it fetches the mail from the pop3 account and stores it all locally on your system, then when your real mail client (I use eudora) would go out to get the mail from the server, it would intercept it and feed the email client the email, which was already filtered. You could also use it as a basic email reader... but I wouldnt suggest that.
The only reason I stopped using it was I was getting some false positives... BUT it did catch about 99.5% of the spam, and just about all the stuff you wouldnt want a kid to see.
There's also supposed to be a service thats like $2.00 a month or something? SpamHeaven or something like that, I saw it on TechTV a long time ago- and it intercepts your email from your regular pop3 or web based server, and filters it for you and they do all the maintenance, adding spammers domains, certain catch phrases, etc... I can't remember the exact name but I'd bet if you search on techtv.com or thescreensavers.com you'll find the service name and/or link pretty easilily.
Personally what I do right now, is between Comast filtering out some, and my permanent email domain provider (pobox.com) filtering out, it gets about 90% or so, and I've setup some filters in Eudora to catch what trickles through... Nothing is perfect, so I hope this helps