There are a multitude of free rulesets available for minis sci-fi gamers, but most of them are generic games such as Full Thrust or Hordes of the Things. Those are great games, but sometimes people want to play a set of rules that is intertwined with a particular setting. I recently became aware of two such rulesets, free downloads for a pair of specific, well-known intellectual properties.
First, and arguably the most recognizable to non-gamers, is Federation Commander: First Missions, based on the original Star Trek television series. Published by Amarillo Design Bureau, FC is actually a leaner version of the company's venerable Starfleet Battles, which although very popular takes a very long time to play.
The free version includes the basic rules at the Fleet Scale level, which is about half as detailed as the Squadron Scale rules. The download also comes with printable counters, two ship system displays (Federation Constitution-class heavy cruiser and Klingon D-7 battlecruiser), and a hex map. So for the cost of paper and ink for your printer, you have a quick-play version of the ship-on-ship battles like Kirk & Co. used to have. For people who gave up on SFB as too esoteric, this game might give you your Trek fix for a lot smaller time investment.
BattleTech Quick-Start Rules, which has 24 pages (8 taken up by in-game fiction), publisher Catalyst Game Labs also offers more free background material, the BattleTech Universe Book, which is twice as many pages as the game rulebook.
This version of the game includes the quick-start rules that cover battlemechs, tanks, and the poor bloody infantry (conventional and power-armored), the appropriate mech/vehicle/infantry record sheets, a color hex map, and lots of counters. If you've been out of the game awhile and want an easy way to get back into BT, I can't think of a better means than this.
So if you've always wanted to wargame in the Starfleet Battles universe or the BatteTech setting (or just want to sample some well-known rules), here's your chance to do at no cost.