Does anybody know what other commercial games have a play by email option?

not really in-game pbem capability, but risk2 from microprose is perfectly playable if all players can agree that player #1 does the positioning for everybody and saves after his turn. The saves have about 6-7 kb, which makes it suitable for modem players.

Age of Wonders: Shadow Magic. The PBEM works ok although there are several limitations compared to the "real game". I played a bunch of AOWSM PBEM (both 2-player and multi-player games) before I sort of lost interest.

I also know people who play HOMM3 and HOMM4 PBEM, although I've never done either.

And there's Dominions and Dominions 2.