InnoDB with clustered indexes is pretty hard to beat for many scenarios. I hate that Oracle owns it, but damn it works well. Also frequent cascading deletes are surprisingly fast, especially in high lock contention environments. [As a side note, it's unfortunate that Wordpress still uses MyISAM by default]

Redis has also been incredibly fast for our ad network. We throw around 7000 qps at the thing with a low number of writes and we haven't restarted the daemon for months. [Thanks Salvatore!]