There are some alternatives, i.e. where Captchas are only displayed when JavaScript is disabled. But all these methods don't work, at least someone just can analyze the traffic and do all the JavaScript-Stuff in a single HTTP/TLS Request, because it's not truly server-side.
I know it's embarrassing to use Captchas, but at least for the registration it should be used.
Or you could use OpenID or some social network oauth system to avoid Captchas.
I've been running all the comments and posts on one of my sites through Akismet (<a href="http://akismet.com/" rel="nofollow">http://akismet.com/</a>) instead of requiring captchas. It works great at filtering out spam but couldn't replace captchas in every situation. For example, I don't think it'd be very effective against fake account signups.
I build my own. You can see it at <a href="http://elbil.forum24.se/humantest.php?todo=%2Fprofile.php%3Fmode%3Dbli_medlem" rel="nofollow">http://elbil.forum24.se/humantest.php?todo=%2Fprofile.php%3F...</a>. Works great.
We have implemented Tic-Tac-Toe<p><a href="http://demo.axiomnext.com" rel="nofollow">http://demo.axiomnext.com</a><p>Click on login to see it.<p>Sorry forgot the source URL from where we took it.