Well I don't know if you know this but SEO is part of SEM. The 2 most popular form on SEM are: 1. Search Engine Optimization (SEO) 2. PPC (Pay-Per-Click) There are other forms of SEM but the 2 above are the most common ones and main ones.
yeah PPC is quite popular, but affiliate programs have been popping up like crazy throughout the past 5 years or so. CPA surveys are becoming quite large as well.