Yeah, none are perfect by any means, but here are some that are generally pretty good (I'm sure I'm forgetting a few; I left a few that are unlikely to be relevant or just don't contain much info out on purpose):
-LiveAquaria (hobbyist oriented)
-Saltcorner (hobbyist oriented)
-JungleDragon
-FishBase/SeaLifeBase
-Fishes of Australia (Museums Victoria)
-Mexican Fish
-Florida Museum of Natural History
-Indian River Lagoon Species Inventory
-Shorefishes (Smithsonian Tropical Research Institute)
-Seaunseen
-WildSingapore
-Kwajalein Underwater
-NCFishes (rarely pops up, but good for some species found on the Eastern US north of Florida)
-INPN (it's French, but comes in handy sometimes)
-Frogfish.ch (frogfish specific, but by far the best frogfish technical info site around)
Edit: New I'd forgotten a few:
-MarineBio (Marine Conservation Society)
-Reef Life Survey
-The Online Guide to the Animals of Trinidad and Tobago (I'm not sure if you can purposefully search through these files on their site somehow, but it comes up pretty often with Google searches - it's species profiles compiled by The University of the West Indies at St. Augustine, Trinidad, and Tobago)
-Eprints@CMFRI (similar to the above, I can't get it to let me search their library on their site, but it comes up pretty often with Google searches - there are a number of species profiles compiled by the Open Access Institutional Repository of Central Marine Fisheries Research Institute)