Are there any alternatives to recaptcha.net, for stopping spam?

CAPTCHA will reduce your spam but it won't eliminate it. People are paid to decipher those glyphs. Some sites use the glyph that was presented to them for their own site so some hapless visitor will decipher it.

Just so you're aware that it's not a perfect solution.

Based on the principle of don't solve a problem until it's a problem: is spam a significant problem on your website? There is something to be said for not annoying your customers/visitors. Even here I sometimes need to make a few edits and I get the irritating "I'm a Human Being" test on typically the last edit I need to make. It's annoying.

People have proposed all sorts of other methods for dealing with this problem. One I read about used picutres of cats and dogs that you had to classify because apparently there's a database of 30+ million of these in the US for abandoned animals or somesuch. This or anything that gets in widespread use will be defeated.

The biggest problem with spam on sites is if you use software that's in widespread use (eg phpBB). Your best bet for those is to make enough modifications to defeat out-of-the-box scripting. You may get targeted anyway but spamming is a high-volume low-success game. There's no real reason to target your site until it accounts for a significant amount of traffic.

The other thing worth mentioning is techniques that can be used to defeat scripted spam:

• Use Javascript to write critical content rather than including it as static HTML. That's a lot harder to deal with (but not impossible);
• Rename and/or reorder key fields like username and password. For example, generate username and password form fields and store them as session variables so they only work for that user. That then requires the user to have visited the page with the login form (rather than scripting a form response that can be POSTed directly);
• Obfuscate the form submission. Things like unobtrusive Javascript that you can do in jQuery and similar frameworks make this pretty easy;
• Include a CAPTCHA image and field box and then don't display them (display: none in CSS). You'll confuse parsers.

The best way for not so popular sites is to insert a hidden field and check it. If it's filled then it's spam because those bots just fill in any field they find.

Add a non-standard required input field. For example, require a check-box that says "check me" to be checked. That will defeat any automated scripts that aren't tailored to your site. Just keep in mind it won't defeat anyone specifically targeting your site.

You might want to look into Akistmet and/or Mollom.