We have played Tigris & Euphrates, and it works great, but would be looking for something different.
Thanks.
How many of you are there? How much complexity and/or depth do you like? At the deep (deep) end, 18xx and specifically 1830 plays well as PBeM (with a Java app called Rails), or there are games like Here I Stand that play well PBeM, or war games. Pretty much anything which doesn't need much interaction between players during each person's turn works well (especially if you can trust each other to roll dice for each other where needed).
If you want some more suggestions, feel free to drop me an e-mail and discuss further.
We stick to 1-hour games mostly because of the lunch hour restriction, if you're spacing the game out over a series of days it might be more interesting to play some of the longer games since it doesn't matter how long you take anyways.