The-Wolf has given a good list there. Wildwoods is another worthwhile place for gobies. They actually have some nice Chinese coldwater ones at the moment. Rhinogobius of some sort, but which ones, I don't know. Call and ask Keith.
More generally, Stiphodon and Awaous gobies are pretty widely sold. Stiphodons can be very colourful (some nice metallic green and blue ones) but they are fish from fast-flowing waters, so think hillstream loaches rather than community fish. Not-too-warm, lots of filtration, and tonnes of oxygen. Awaous gobies are easy to keep, but short-lived (two years seems the max). Adults are just not worth buying, go for the 2-3 cm youngsters. That said, these are really lively, friendly fish. Mudskipper-like, they will crawl onto your hand (underwater, obviously). My girlfriend's favourite fish, and she's been in mourning since my old, old male died at a geriatric 18 months or so. All Stiphodons are freshwater fish, but Awaous gobies have a complicated life-cycle with the eggs floating down into the sea. Stiphodon and Awaous are around 10 cm when fully grown, often smaller.
Violet gobies are very fashionable, and great fun, but being big and demanding fish, they aren't for everyone. Ditto mudskippers, which are gobies in good standing. 90 litres is probably a bit too small for keeping adults of these though. Mudskipper in particular are very territorial.
Don't forget sleeper gobies. There are quite a lot of Eleotris and Dormitator species traded periodically. Maidenhead Aquatics in St. Albans had some Eleotris sp. (perhaps E. fusca) in stock as "brown gobies" or something similarly meaningless. Dormitator lebretonis is a great fish, very active and pugnacious without being dangerous. A group of these in a planted, slightly brackish tank would be fun. Butis butis is another small-ish sleeper goby (~15 cm); territorial, predatory, but otherwise pleasant enough. Most Eleotris and Dormitator get to around 10 to 20 cm but there are some giants so check before purchase.
I think gobies are set to be the next big thing. There are literally thousands of them, and most true gobies (as opposed to sleepers) are either small or very small fish. So you can take a gamble on "mystery gobies" without much harm. Keep any you don't recognise in slightly brackish water; freshwater species won't mind, and the brackish ones will need the salt.
Cheers,
Neale
