These camera makers' full-featured accessory flash units offer variable flash output (flash exposure compensation) on TTL auto, as well as manual flash with adjustable levels. They are compatible with current digital and film SLRs from the respective manufacturers. But they are not necessarily backward-compatible with older SLRs, nor will older flash units necessarily work with current cameras. Check the manufacturer's specs carefully before you buy.
Canon Speedlite 580EX II ($405, street)
Nikon Speedlight SB-800 AF ($315 street)
Olympus FL-50 ($365, street)
Pentax AF540FGZ ($335, street)
Samsung SEF-54PZF (discontinued, available at some retailers for about $275, street)
Sigma EF-530 DG Super ($260, street); EF-530 DG ST ($190, street) is identical but lacks wireless TTL operation.
These accessory TTL flashes that can be used with other manufacturers' cameras. As above, do not assume compatibility; check to make sure a flash works with your specific make and model camera.
Metz 58 AF-1 for Canon ($375, street) and Nikon ($350, street).
Nissin Di622 ($170, street): models for Canon and Nikon.
Sigma EF-530 DG Super ($260, street) and EF-530 DG ST ($190, street) for Canon, Nikon, Pentax, Samsung, and Sony.
Sunpak PZ 5000 AF ($150, street): models for Canon, Nikon, Pentax, Samsung, Sony.