Initially created by Justin Mason in 2001, the open-source SpamAssassin spam filter is pretty widely used. It's not in use directly at the top tier of ISPs like Yahoo, Hotmail and Gmail, but it does seem to be used by various second and third tier providers, B2B sites, hobbyists and educational institutions. And even though this filter isn't exactly the same as the filters in use at, say, Gmail, there's likely some truth to the theory that "what SpamAssassin suggests may be spammy, other filters may find spammy as well." Great minds think alike, to some degree, the theory goes. Thus, some folks find SpamAssassin to be a valuable tool to check to get some idea if their email messages might be considered spammy.
Want to check your email against SpamAssassin? Postmark has this tool where you can copy-and-paste your message headers and content, or use an API call, to check a message against the SpamAssassin filter. SpamScoreChecker.com will give you an address to send an email message to, and then allows you to click through to see the scoring results. IsNotSpam.com is another tool that works similarly.