Some feature very high-high automation / with dedicated capabilities to sync to ETTL/II such as Metz, Sigma. Some feature good old manual use, mainly for strobist usage, while others may feature potential cross-connection capabilities (Pentax-Canon, Olympus-Canon, Nikon-Canon).
Take your time and check out the Mass Sellers and other stores. Best is to do almost ALL your work before hand and Read The In-Depth Reviews. They can offer critical advise regarding digital equipment.