Fish for a small tank really depends on just how small the tank is. Dwarf puffers need five gallons each. That means you can keep one in a five gallon tank, or two in a ten gallon tank, but if you try to put two in a seven gallon tank you're going to run into trouble. They are extremely aggressive and so there is a general no-tankmates rule.
Rams need a ten gallon minimum, and the tank must be mature (cycled and running with fish for 3 months plus) because they are very sensitive to water quality. You could have one ram plus some small community fish in a ten gallon, or a breeding pair. Pairs are territorial, so to keep a pair in a 10 gallon basically you couldn't put anything else in with them. The minimum size for a pair of rams with tankmates is somewhere slightly over the 20 gallon mark which probably doesn't qualify as 'small tank' so I won't go on about it. When not breeding they are extremely peaceful, and a single ram can be kept with virtually all community fish.
Obviously the single male betta is a standby for a five-ish gallon tank. If you pick a nice one, they are beautiful, and they've got to be right up there in terms of personality... they have WAY more than most other fish.
Most of the tetras, danios and rasboras are small shoaling fish. Some of the common tetras (namely rummy nose, neon, black neon, glowlight and ember) are good for small tanks. Generally the narrow bodied ones do not grow as large and fit in smaller tanks. The ones that are a bit disc shaped (like the black widow, lemon, serpae etc.) are larger, and often 'nippier' meaning they have a greater tendency to regard other fish's fins as snacks. All tetras will do this to some point but the smaller ones do it less; I've got ember tetras with a veiltail betta (which is technically a big no no) and they do fine together. Embers have to be my pick of the litter for really small tanks... they look amazing as a shoal of about eight in a five gallon tank. This is slight overstocking but works if the tank is well maintained as they reach barely an inch adult size. When you feed them properly, they morph from the colourless, stressed out fish in the dealers' tanks into shining red gems. They are very undemanding fish.
Zebra danios and cherry barbs are also options for small tanks (although avoid the other barbs, which grow large, are obnoxious and need space.) These two will both nip fins, so it's best to keep them in a single-species tank or with bottom feeders only, unless you have enough space to keep a large school of at least fifteen (which reduces fin nipping a lot in most species, oddly enough.)
Micro rasboras (things like Boraras brigittae and B. merae) are even smaller than ember tetras and even more brilliantly coloured. They are more sensitive and harder to look after than ember tetras, and also hard to find and usually quite expensive. In my opinion they're worth it and I would pounce if I ever saw them (although nobody ever imports them into AUstralia, for some reason.) You could also consider the harlequin rasbora and the hengeli rasbora, which are slightly larger, more available, but both suitable to be kept as a school in a small tank.
Bottom dwellers: pygmy corys and otocinclus are both schooling. Pygmy corys are not colourful, but they are attractively marked and very playful fish. Keep at least three or four. Otocinclus catfish are another micro catfish. They are some of the most difficult fish in the hobby. They are hard to acclimatise, hard to feed and extremely sensitive to water quality. They really need the company of their own kind. Do not even attempt to keep them unless you can keep at least six and whatever you do, don't overstock their tank. It must be mature, well and truly mature. Also, they need specialist feeding with vegetables and algae wafers which gets messy, fiddly and expensive. If you're willing to go to the colour and expense, ottos are very unique and attractive little fish.
Even smaller are the hara catfish, which I've seen only online and never kept, can't tell you much about those but they're probably worth researching.
If you can get them I'm always an advocate of the pygmy rainbows: blue eyes (psuedomugil species) and the threadfin (Iriatherina werneri.) Threadfins and P. gertrudae (spotted blue eye) are fine as a school in a well planted small tank. These are the two you're most likely to encounter. P. tenellus (delicate blue eye - which is actually pretty tough, go figure) and P. furcatus (forktail blue eye or occasionally popondetta blue eye, which is a misnomer, the true popondetta blue eye is P. connieae which to my knowledge has never been exported from Australia) crop up from time to time, same deal. P. signifer (pacific blue eye) gets larger (6cm to the other species 3-4) and the males are territorial. It needs space, fifteen gals minimum.
And then there's Endler's livebearer, which is smaller, prettier, not deformed (from breeding the fins into ridiculous shapes and sizes) and far hardier than the common guppy. I recommend all males as getting females will rapidly overstock your tank for you. THey breed like... well, livebearers.
When you start getting up to ten or fifteen gallonish all the other livebearers become options. So do some of the smaller corydoras catfish (the panda for instance) and some of the larger tetras and barbs. If you tell us exactly how big the tank is we can probably work a heap more stuff out.
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