I have yet to see a game that approaches the simple elegance of GDW's Triplanetary.
The primary thing that Triplanetary offers is a really cool movement system, which was later used (in a different form) in GDW's Mayday. The scale is approximately daily turns, with the solar system from Jupiter inwards represented on the map. [According to former GDW employees, the name had no particular meaning other than evoking "space-like feelings."] The different parts of the map are clearly not to scale -- both the distances between objects and the sizes of the objects themselves are somewhat inconsistant (although an argument could be made that, at least with the distance distortion, the map represents required delta-vee changes rather than actual distance).
Movement is Newtonian -- once you are moving along a certain vector (speed and direction), you continue moving along that vector until acceleration (in the form of gravity or thrust) is applied to change the vector. Vector arrows are drawn on the laminated map with AV markers. Each ship has a basic thrust value of one, meaning that it can move the head of the vector arrow into one of the six hexes surrounding the hex the ship will move into if it simply coasts along its current vector. [Some ships can thrust two, but they must have a maintenance stop afterwards before they can do it again.] Each thrust point costs one fuel point; most ship types have between 15-25 fuel points, with a few having more and a few less.
Major gravity wells (the sun and planets) are surrounded by six heavy black arrows (in the six surrounding hexes), each pointing towards the planet. A ship or other object (e.g., torpedo) entering the hex is accelerated 1 hex in the direction of the arrow the turn after it enters the hex. If you think about it for a moment, you realize this is exactly what happens when a body is in orbit -- its current vector would take it off into space, but it constantly gets an acceleration which causes it to fall around the planet. This is one of the nice things about this game -- rather than the typical "I'll spend a thrust point to go into orbit", you have to actually make an approach, decelerate to an orbital speed, and then burn to actually enter orbit. Once in orbit, the game mechanics will keep you there indefinitely.
There are also minor gravity wells, which have hollow arrows. With hollow arrows, you can either take every other one as a full gavity arrow, take one of every two, or ONLY take every other one -- the different editions have slightly different rules, and I know people who modify it with house rules as well.
Combat occurs in the form of guns and ordnance (mines and torpedoes). The gun procedure involves a simple formula which includes the distance between the two ships and the relative velocity (which is determined by drawing the two vector arrows base-to-base and counting the number of hexes between the arrow heads). Mines and torpedoes are launched at the beginning of the turn, and move in the same way ships do. Mines are not very powerful and have no independent thrust capability, but take up only half the mass of torpedoes, so you can carry twice as many. Torpedoes are deadly and have one thrust point. In practice, gunfire is rare, some people like mines, most people opt for torpedoes.
Overall, Triplanetary has the best rule length to result ratio I have seen in any game. The scenario I have played the most is the race scenario -- players have to touch all the major gravity wells and make it back to Earth. This isn't as simple as it seems: outside of certain zones close to major planets and moons (which I refer to as the "CNN coverage zones"), you can use weapons.
Okay, so what's wrong with it? Well, aside from the map, there isn't much variety in weapons, the weapons you do have are stupid (so stupid that you must change course when you launch a mine, because they have no IFF), the map is somewhat cramped, the grease pencils get messy etc. That having been said, I _really_ like this game, and most of the above complaints can be fixed.
And that, I think, is Triplanetary's real value: while it is an enjoyable game in it's own right, it is an excellent base on which to build your own game. In addition to the campaign overgame idea which has been sometimes discussed on the Net, here are some things I think could/should be added:
1) Variant gravity fields (I made some by drawing planets and stars with their gravity arrows of hex paper, then photocopying it onto 8.5x11 label sheets, then laminating one side of the sheet);
2) Replace the simple "maintenance stop" rules for doing two-hex burns with the concept of "wears" from Avolon Hill's Speed Circuit [In Speed Circuit, you can build extra sturdiness into your design; if you do something beyond the car's capabilities, you expend a wear, and when you are out of wears, you start rolling on the "bad things" table];
3) Redo torpedoes and mines, to add IFF capabilities and (for torpedoes) more complex guidance and maneuvering;
4) Add ECM, ECCM, and sensor rules.
Unfortunately, Triplanetary is currently out of date, although the rights are now owned by Steve Jackson Games, and it is listed as a long-term project.