Most of the spam I get through to my Gmail inbox is from SEO spammers using Gmail. They keep sending the same material with the same username formats and same names, I mark hundreds of them as spam, and they still make it through.

Another large portion of my spam is from Chinese product spammers using Picasa albums and send to a friend feature. There are limited ways you can report this and despite marking almost every Picasa email I've received as spam, every single Picasa email still makes it through.