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Thunderbird has pretty good spam detection / quarantine. In addition v1.5 can now spot potential "Scams" as well such as the all too common PayPal and eBay phishing emails.
If you want something server-side I would check out SpamAssassin.
I use the DCC and the Blacklist filter plugins as the main filters.
The DCC checks how many other people have received the same e-mail. When that number is over an amount you set, the e-mail is blocked. I set mine to 60.
I let the Blacklist filter only use the Spamhaus list. Among the other lists that are preconfigured are some that give false positives in my situation, so I removed them.
Further I use plug-ins to block e-mails that are almost empty or contain only an image, and e-mails that contain an attachment with an extension that is likely a virus or trojan (.exe, .pif, etc).
My ISP has a spam filter on the POP server (Brightmail), but this Spamihilator configuration seems more effective.
[Added:]
Spamihilator has pre-installed filters for keywords and a filter which needs to be trained. I've turned those off.