The basic idea with Alternity was that it was primarily skill-based with some class elements. The main task resolution was rolling d20 ("control die") plus often another die ("situation die") based on difficulty, and wanted to hit your skill value or lower. Skill value was equal to your ability score (which was in the 4-14 range for humans) plus skill ranks (up to 12, no more than 3 at game start). If you rolled half your value or less that was a Good success, and a quarter was Amazing (these numbers were listed on your sheet, so if you had skill total 14 it would be written as 14/7/3). The situation die was normally a 0 if you had the appropriate skill or +d4 if you didn't, and scaled up or down as follows: 0, d4, d6, d8, d12, d20 (either positive or negative). This had the somewhat weird effect that things that were more difficult were expressed as a + modifier, and things that made things easier were expressed as a negative.
You had three different damage tracks: stun, wound, and mortal. Depending on the weapon and success level, you'd do damage to different tracks – a club might do d4+1s/d6+1s/d4+1w, meaning it does 1d4+1 stun on an ordinary success, d6+1 stun on a good success, and d4+1 wound on an amazing success. Dealing wound or mortal damage also meant dealing half as much of each lower category (so 4 wounds would also deal 2 stun). Armor reduced damage, though not secondary damage (so if you hit for 4 wounds, and it was reduced 3 points by armor, you'd deal 1 wound and 2 stun). There was also a fourth separate track for Fatigue, but it didn't interact with the others.
Characters were primarily skill-based. You had classes (combat spec, free agent, tech op, diplomat, with an optional mindwalker), but they mainly boiled down to a discount on certain skills and one or two special abilities, plus they determined what level you could get various perks at. Having high skill levels also often came with various special abilities/bonuses. In some cases, you could spend skill points to get early access to these.
The game itself was setting-neutral, but had two main settings. The first was the far-future space opera Star Drive, which had the stuff you'd expect: spaceships, multiple stellar nations, a fair amount of different species to play, invaders from Beyond, and so on. The other was the modern-day Dark Matter, a conspiracy-themed setting strongly influenced by X-Files. There was also a one-off Gamma World book, and I think one of the bigger computer games at the time (either one of the Fallouts or Starcraft) had an adaption included on the installation disc.
I would think Alternity is available at DTRPG, but I can't be hedgehogged to check.