I wish I could share the one I use at work because it stops 99% of spam emails... but I helped design it, and sell it to clients, so I can't.
One that I have been testing on my personal computer is spam inspector. I can't call it all that effective. I've had it for probably 3 months, and the only spam in the spam box is what I've manually put there to train the filter. It does have a nice feature which allows me to "bounce" the message back manually. So, if I suspect that someone will see it, I bounce the message and they remove me because they think it's a bad address. Another nice feature is that I can block all email from a certain sender's domain if I want. I like THAT one because it DOES work! For instance, I tell it to block all email from Yahoo, hotmail, and MSN (common addresses people use to spam) and then put friends on the allow list. That part works great too.
Now, if I could just get the heuristics educated enough to stop all the spam, it would be a great program!
I consolidate all my mails through Mailsnare and find their service excellent. They have a great choice of customizable filters, blacklists etc. Their customer service is second to none. Runbox also has a good service and I use them as a backup to Mailsnare.
K9 is pretty good, sits client side, and once you've 'taught' it how to behave it gets it right 99.45%(at present) of the time. Almost all of these are spam getting through (false negatives, 0.52%), rather than 'good' (false positive, 0.03%) emails being marked as spam.
I use SpamCop to aggregate all the addresses I have collected over the years on clients servers. The SpamCop server collects my emails from clients servers and everywhere I have an email address, washes it with the filter settings I have set and forwards what's left to me at my business email address. Average this month is around 500 emails a day of which about 465 are spam. One or two false negatives a day and only two false positive ever.
My current filter settings on spamcop are:
spamcop Blacklist bl.spamcop.net SPEWS level 1 spews.relays.osirusoft.com Osirusoft open relay inputs inputs.relays.osirusoft.com ORDB open relays relays.ordb.org Spamhaus Blacklist spamhaus.relays.osirusoft.com South Korea korea.services.net Osirusoft Open Proxies socks.relays.osirusoft.com monkeys.com open proxies proxies.relays.monkeys.com China cn.rbl.cluecentral.net Nigeria nigeria.blackholes.us Argentina argentina.blackholes.us Brazil brazil.blackholes.us And SpamAssassin Limit 5
It costs $30 a year and I would still use it if it cost ten times as much.