On: You probably don’t need ReCAPTCHA

[nitrogen]

The spam I see is trying to add little bits of pagerank all over the place.

[dorgo → nitrogen]

What about the wikipedia solution for this: rel="nofollow" ?

[JoshTriplett → dorgo]

The spammers don't care; they spam anyway.

[luckylion → nitrogen]

That also scales only if you don't customize. A low-value backlink that will usually be removed in the near future isn't worth an hour or two of a developer's time.