Elsewhere, someone suggested Solvemedia, but I think that all captchas based on reading text are actually easier for high-quality OCR to solve than for humans. The photo-based ones give computers a major disadvantage over humans. I don't care very much if people use services like 9kw.eu to solve the captchas, since that at least has a cost, but if a captcha can be OCRed, then they have no cost at all, making them completely useless.

If someone has other suggestions for good captcha services/libraries, let me know. I don't really like using Google products, since Google's whole business model is spying on people. Though as I mentioned, I don't have much faith in captchas based on reading mangled text.