A big flat screen. Indianapolis had both curved (Children's Museum) and flat (Indiana State Museum) IMAX theaters for a while, but the Children's Museum got rid of theirs and put a dinosaur exhibit in the old dome.
The dome-style IMAX are actually apparently quite rare, the "normal" IMAX is just a really big (about three stories tall) screen, that is slightly curved usually.
I believe Six Flags over Georgia had an actual IMAX dome that I remember as a kid - a projector experience of short films such as flying airplanes, racing cars and such.