A few of my fish have interesting habits:
McCulloch Rainbows: Will often display to each other, then will swim in tigh circles around each other with all fins erect.
Harlequin Rasboras: Swim backwards sometimes - a few of them will get together and will repeatedly swim backwards slowly for a bit, then dart forward very fast. Watched them do this for 30 minutes one night.
Siamese Algae Eaters: Often swim around together in parallel, and then will sart feeding on algae in parallele too. Cool to watch.