I don't do a lot of sci-fi gaming, and when I do, I tend to play 6mm. However, over the last couple of years, an increasingly the last few months, I have been working away at three forces for sci-fi skirmish games using 15mm figures. I haven't decided on a rule set yet, but Grimdark Future: Fire Fight seems to work very well so far.
These engineering space dwarves from the Nidavellir system are led by Sgt Grimm. They. Are. Absolutely. Class. Wee. Sculpts. These are 15mm versions of the Onslaught 6mm Grudds, both available in the UK via Vanguard Miniatures. They stand about 11mm to the eye, so they are noticeably shorter that human sized figures, but they are about twice as wide.
The last faction are a cell of smugglers who may or may not also be rebels led - either way - by a mystic space wizard with a laser sword. They come from a variety of sources and have little in the way of uniformity, so I ensured that each of them wears something sky blue to act as a force identifier.
Highlander Studios, Space Pals.
Ral Partha Europe, Critical Mass Mercenaries.
Ground Zero Games.