Akismet is probably the perfect example of a free plugin.

If I remember correctly, almost everyone uses it, fully-featured, for free but if you have a massive site and subject the service to unusually high usage, you pay a few dollars, but far less than it would cost you to police your own comments.

Again, Akismet is one of the things that has allowed WordPress to bloom, to everyone's advantage, and now people are complaining about it?