See
Reading Adobe ePub on the Desktop for three alternatives all based on mobile Adobe Digital Editions. I like NOOKstudy the best. There is also a new version of Desktop ADE which is a big improvement over the original, see
Adobe Digital Editions 1.8 preview now available.
For maximum flexibility, it is hard to beat
FBReader. It allows you to adjust practically everything about the reading experience. It does not fully support ePub CSS though. In my experience ePubs are always readable but some formatting and graphics may be lost.