3D napkin sketches.
Sets of fleets I did in my copious free time, usually spending 1 to 4 hours per ship, with a few taking more than a day. They make beer money on Shapeways.
Sometimes it's harder to come up with background ideas than it is for the ships themselves.
Earthers. Modular ships for quick production. Fond of gunboats over fighters. The rings on the aft end control the magnetic plasma sails needed to get around the solar system.
Belters. One of my first actual fleet sets.
The Martian Colonial Space Force. More of a defensive, stay at home type fleet.
The USA's Orion Drive fleet. Unsung heroes of the secret space wars of the 70s.
Russian Orion Drive fleet, 70's vintage. You never hear about these ships any more.
Privateers. Old-timey war rockets in the hands of greedy, evil and often smelly men.
The United States Aerospace Force. (I needed to call them something...)
Federation of Man. A small coalition on the edge of a fallen empire, spoiling for their share of the carcass.
The Imperial Spican Navy, eternally shining jewel of might against the implacable darkness.
The Buru Authority is non-expansionistic, but aggressively territorial. The Buru themselves are enormous, armored omnivores, quick to take offense and slow to trust.
Rabbits. Actually human, but called that for their cybernetic antennae. A low-rent bargain basement Borg with very maneuverable ships.
The sleek and powerful Novus Regency Space Navy. Featuring advanced technology bordering on the miraculous, and mighty stylish to boot.
The Ikennek Regime. Brutalist Starblazers.
Chukulaks. Big, polar-bear sized parrots. Fun to pal around with on their terms, but they won't invite you over for coffee.
Reticulans. No outward signs of intelligence apart from their scary technology.
Colonials. Loose riffing on the BSG look.
Eltanni. Different riffs on the BSG look, and designed for those who like smaller scaled ships.
Stravok Invasion Force. Jovially disrespectful of borders of any kind.
Gee, I wonder what inspired these?
The Terran Union (updated). Hardcore defenders of their smattering of worlds who take themselves way too seriously.
Galimeks, Another small scale fleet, inspired by Traveller's Type-Y Yacht.