I've been looking around a bit for games similar to the Descent series, but can't seem to find anything, but one game called Forsaken, which my brother bought a decade ago (pretty good game if you are looking for something similar as well).
Basically what I'm looking for is the same type of gameplay. Full 3D environment, freedom to move in all directions, all taking place in same style of maps as the Descent games.

G-Police, Terminal Velocity and Forsaken are pretty much the only thing that pop in my head right now. There have been alot of games similar to G-Police, especially on the PlayStation. I could suggest titles like Omega Boost, but they mostly become space sims (like the FreeSpace series) and are probably not worth mentioning in this thread?
Though Forsaken of those must be closest, since it's mostly situated underground and has a true 360 degree freedom like Descent.

Space sims suggestion I think would be more fitting in the Freespace forum or general discussion.
Anyway, I've got Terminal Velocity, which is pretty cool, and I checked out G-Police, which looks pretty cool, so I'm going to try to check that one out.

I remember playing a demo of G-Police on a PC-Gamer demo disk back in like 97.
Wasn't it a Psygnosis game?
And it was cool. Very cool.

Another one that's somewhat similar is Hellbender by Microsoft.
http://www.youtube.com/watch?v=mUeBo6g5gZY&videos=vpuZyBEi0A4&playnext_from=TL&playnext=1
But, it's closer to Terminal Velocity than anything, so there's no strafing or vertical shifting on the fly like in Descent/Forsaken.

Looks pretty cool. I suppose it has the same engine as Terminal Velocity, as the graphics and even music seems very familiar. And the crosshair looks like it was taken directly from TV.

Not surprising as they were both developed by Terminal Reality. Also check out Fury3. Hellbender is the sequel to Fury3.

I also seem to recall a DOS game by Epic Megagames called Radix: Beyond the Void.
It wasn't exactly like Descent (you were always moving forward, for example) but it always struck me as similar to Descent.

Radix was very similar and even released the same year IIRC. However, the way you moved in Radix was closer to Doom. I don't think it supported true 360 degree play.
is said to be very similar to Descent. There was also [url=http://www.mobygames.com/game/windows/defiance]Defiance, but that was closer to Quake in a hovercraft from what I remember. Both are worth a try, I'd say.

Weren't Extreme Assault helicopter levels similar as well?

I know it's been quite a while since you made this post, but I figure since the game was so old when you wrote this, perhaps you're still interested in alternatives. And since this post comes up on the top results of a "games like descent" Google search, it may help others as well.

I think Miner Wars 2081 just may be what you're looking for. It's the closest I've seen to a modernized decent. Important features/facts to note:
1. The lead developer admits to being a fan of descent--and as a result, this game is inspired by descent.
2. 6 Degrees of Freedom (6dof) shooter game.
3. Immersive singleplayer campaign (with Co-op option), and Multiplayer mode.
4. Underground tunnel, and space environment
5. Features persistent, fully destructible environment. (Ie., tunnel your way through an asteroid. Through there are also indestructible things).
6. Varying types of missions: "rescue, exploration, revenge, base defense, theft, transportation, stealth, search and destroy, pure harvesting, racing or just flying around and destroying everything you see." (ign review).
7. Can harvest and sell ore.
8. Huge game world, with billions of sectors.
9. Wide Variety of Ships (and upgradeable ships)
10. Epic-Scale Battles (up to 300 ships)
11. Looting
12. Factions

It's currently in beta stage. and has been greenlit by steam. Also... for a limited time, you can buy it at half price (It's still in beta, but they have finished several of the missions). But if you don't want to buy it, you can still download it an try out the demo (1st mission). Download link is here:
http://www.minerwars.com/Downloads.aspx?aid=zaptower

The other game that's out there is Retrovirus

It's AWESOME and the developers are huge Descent fans :)

Forsaken was released as freeware. Now it's called ProjectX.

I'm looking for an old game (from the late 90s I think) which was some sort of mixture between decsent and jungle strike in the way that you played in a spaceship that flies around looking for soldiers on the ground and then turn Into a land veihcle and pick them up
While doing it you also had to take care of your fuel so you had to use the soldiers you rescued to go and pick the fuel tanks from the ground
Last thing I remember about the game is when you want to move on to the next level you fly into a spiral gate and it takes you to another zone

Not the answer for last post, but a very good multiplayer game made all alone by one guy, that was inspired by Descent 3 (multiplayer).

Talon
http://www.talongame.net/en/