SpamAssassin is this wonderful unix app that checks each email coming into my mailbox to see if it looks like spam.  Basicly it's got a list of common attributes that make up spam mail and each one has a certain point value.  If a message has a high enough score it marks it as spam and moves on.  I then have procmail filter those messages to their own folder.

The best thing about it though is some of the things that count as hits.  And every once in a while it seems like you get a message that just seems to hit just about everything that can be hit.  Here's the results for a particularly spammy message:

SPAM: Content analysis details: (36.84 hits, 5 required)
SPAM: Hit! (1.2 points) From: does not include a real name
SPAM: Hit! (2 points) Invalid Date: header (no timezone)
SPAM: Hit! (1.31 points) Character set doesn't exist
SPAM: Hit! (1.77 points) BODY: No such thing as a free lunch
SPAM: Hit! (3.32 points) BODY: Doesn't ask any questions
SPAM: Hit! (1.57 points) BODY: Talks about direct email
SPAM: Hit! (1.6 points) BODY: Offers a full refund
SPAM: Hit! (1 point) BODY: Asks for a billing address
SPAM: Hit! (1 point) BODY: Asks you for your signature on a form
SPAM: Hit! (0.7 points) BODY: Contains word 'profits' in all-caps
SPAM: Hit! (0.5 points) BODY: Contains 'for only' some amount of cash
SPAM: Hit! (1.72 points) BODY: Talks about email marketing
SPAM: Hit! (3.46 points) BODY: Contains word 'AMAZING'
SPAM: Hit! (1.2 points) BODY: HTML mail with non-white background
SPAM: Hit! (1.55 points) BODY: Contains an ASCII-formatted form
SPAM: Hit! (1.56 points) Contains phrases frequently found in spam
SPAM: [score: 12, hits: all you, are you, broadcast]
SPAM: [email, check credit, city state, com subject,]
SPAM: [com with, credit card, credit cards, email]
SPAM: [address, email advertising, email from, email]
SPAM: [package, find out, for free, for only, for your,]
SPAM: [free bonus, free order, from our, increase]
SPAM: [sales, number exp, order form, order phone, our]
SPAM: [broadcast, phone number, product service,]
SPAM: [receive least, response rate, send out, state]
SPAM: [province, thank you, the internet, this email,]
SPAM: [this program, united states, web site, what you,]
SPAM: [when you, with our, with your, within days, you]
SPAM: [for, you not, you receive, you you, your]
SPAM: [business, your email, your name, your]
SPAM: [order]
SPAM: Hit! (3.33 points) HTML-only mail, with no text version
SPAM: Hit! (1.75 points) From and To the same address
SPAM: Hit! (1.8 points) No MX records for the From: domain
SPAM: Hit! (2 points) Received via a relay in relays.osirusoft.com
SPAM: [RBL check: found
225.159.1.212.relays.osirusoft.com.]
SPAM: Hit! (2 points) Received via a relay in relays.ordb.org
SPAM: [RBL check: found 57.62.43.216.relays.ordb.org.]
SPAM: Hit! (0.5 points) Received via a relay in ipwhois.rfc-ignorant.org
SPAM: [RBL check: found
225.159.1.212.ipwhois.rfc-ignorant.org., type: 127.0.0.6]
SPAM:

I highly recommend this app to anyone who's a unix mail user.  It even runs on my OS X box.