Community Wiki: How can I prevent comment spam on sites which I control?

The topic of spam prevention comes up fairly often, but the answers which are provided aren't consistent (because everyone has his or her own favorite solution).

To ensure that askers see all the options at their disposal, I've put together this catch-all question (note that questions about using specific spam prevention tools should still stand on their own - this is more for "what should I do about spam?" questions).

Please take this opportunity to contribute your own spam prevention solutions/recommendations and vote up this community wiki Q/A so askers have a chance to see it before they post a "what should I do about spam?" question.

Good work. Because it's community wiki you don't get the appropriate reputation for your work, so thanks. –  paulmorriss Feb 2 '12 at 13:33
@paulmorriss I don't think it would be appropriate to create a catch-all that isn't community wiki, but I hope to create more of these in any case (feel free to suggest or, if you have time and see a need, create a catch-all for a topic that interests you) –  danlefree Feb 2 '12 at 15:14
@danlefree You did a great job and thanks for including my solution –  toomanyairmiles Feb 3 '12 at 21:43
@danlefree would it be a good idea to link to plugins that will accomplish these techniques in popular software (like wordpress or phpbb)? –  Hamlet Feb 4 '12 at 16:29
@Christofian Certainly - probably most useful to create lists at the end (e.g. under a separate <h3 /> for each major CMS) –  danlefree Feb 4 '12 at 21:12
@danlefree also, a lot of people are creating new answers to this question instead of editing the existing answer. Is there any way to stop this? –  Hamlet Feb 4 '12 at 21:49
@Christofian I don't believe I can lock new answers without locking edits on the existing question and answer - no worries though, the extraneous answers can be sorted out with a comment or two. –  danlefree Feb 4 '12 at 23:37

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