2 of my favorite alt histories -
1. Ars Magica. Have to agree with the above poster, truly excellent system, and one which leads to having games that truly go to epic campaigns. Current game going 1.5 years.
I like to explain it like this: Europe in 1220 where the myths are true. Angels save, demons corrupt, faeries try to trap you and magic exists. Great setting, though some of the extra books are a little hit/miss.
2. Godlike: Great fun! Supers game in WW2, focus on the WW2. Could be run as a normals with teeny powers game or a higher power level. Expect some deaths.