A Betta Bowl: No filter required, a male Betta with like colored gravel and a single plant. Makes a great display
A pair of Endlers Livebearers in a one gallon jar. This jar uses an under gravel filter and is planted with Java Moss.
A Seahorse Corral: Add an undergravel filter with aragonite gravel and a small live rock, get yourself a pair of dwarf seahorses. It's an interesting and novel aquarium project.
A Killifish Tank: A small pair of Killies would do fine in a one gallon bowl. Add some peat moss for the substrate and some Water Sprite for floating cover and you might even get some fry.
A Bumblebee Hive: Add enough rocks to half fill the bowl, stack them to make as many little caves as possible. Add about 3 Bumblebee Gobies and now you have a Bumblebee hive.
A Guppy Gallon: Start with a pair of Guppies and an undergravel filter, add some Water Sprite and watch the little family grow.
A Dwarf African Frog Pond: Put a few Dwarf Frogs and let the fun begin.
Not for Goldfish
The possibilities are almost limitless, but in a container as small as one gallon, special considerations must be made. The water will need to be changed more often than with larger tanks, temperature fluctuations must be controlled, just keep the bowl away from windows, heaters and air conditioner vents, and remember the population has to be kept small.
But one very important point to mention is that a one gallon bowl is NOT a good home for a Goldfish. For tips about Goldfish please see The Golden Rules - Goldfish Guide.
Comment By: Chris No offense ment to any of your fish keeping skills however the idea of a "Seahorse corral" is probably not the best idea. 1. Keeping the saltinity in a 1 gallon bowl would be near impossible, 2. Dwarf seahorses are extremely expensive in most places and should not be in a 1 gallon bowl. If you have a correction to anything I have said I would be happy to hear it and use it in my own fish keeping. Thank you
Response By Clint: The seahorse corral would be for Dwarf Seahorses only. They are cheap, easily raised by home aquarist and do well in small tanks. See http://www.seahorse.org/library/articles/dwarfKeeping.shtml for info about these interesting little ponies. You can also google for "Dwarf Seahorse" and find several sources for dwarfs at $5.00 each. Thanks for the excellent comment/question and your concern.
Comment By Gold Comments: I would also really like to see A 'Ghost Shrimp Colony' added to this. Just get a One Gallon, insure it has a lid, put some sort of cave in the middle, a bit of java moss/fern. Add two-four Ghost Shrimp, and hey-presto you have you ghost shrimp colony. Ghost shrimp add very little to a bio-load, and eat absolutely anything. They require no special needs, probably the easiest 1 gallon you can keep!