I've used Spamcop for many years, and it has an excellent success rate.
Any of my e-mail addresses that starts getting spammed is then filtered through Spamcop (there's no limit to the number you can do this with). The "clean" messages are forwarded on to a clean address, and the Spam stays on the server till I visit and delete it.
The clever bit is a feature called "SpamAssassin" which looks for key trigger words and scores the e-mail accordingly. A score of 1 is not a spam, a score of 20 certainly is. You decide the threshold you want to use - I chose 3.
There are other filters (I use them all, but SpamAssassin is what catches the great majority). There is also a "whitelist" for those folks who choose to use hotmail and such mailboxes - and for the blueyonder subscribers (BY seems to be blacklisted more than it's not, these days).
I get about one spam a day leaking through, and anywhere between 200 and 500 a day trapped. False positives (genuine messages caught in the filter) are less than one a month.
All for $30 a year. Excellent value, I say.