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Image recognition - it's quite a bit of a cottage industry for spammers. Most CAPTCHAs for common software packages (such as vBulletin) or popular sites have been broken. The best thing I've come across is a "prove you're human" field in the signup section. Basically, it's a custom field where you ask a question that any human could answer, say "What's 4 x 4?", and check to make sure the answer is "16". But the idea is that it's different for almost every forum, so there's no way to programmatically defeat it like they do with common CAPTCHA systems. Unless you have a site that people are specifically targeting, it'll basically make you immune to robots.
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