Olympus' own FL-36 is a pretty capable, yet small(ish) flash that complements the cameras quite well. The FL-14 is also a reasonable flash (much smaller) but large lenses can cause cut-off. I should add, defining what sort of things you intend to use the flash for, will assist in informing what might suit. One person's idea of a travel flash might be something the size what comes standard with the E-M1, another's might be an FL-50R.
Thanks, OzRay, Agreed on the need for more specifics. I would need the flash for indoor shooting of classroom activities, maybe some field work in tropical forest type settings but I do not want to go too big. I will check out the FL-36. I think the FL-14 might be a bit small due to the lens issue.