Hi, folks. I recently started playing the X-Com UFO Defense PC game again. It was released on Steam with an improved DosBox version that actually works on my laptop. If you're not familiar with X-Com, this was a great series of PC games from the early 90's, wherein you control X-Com, an organization tasked with combating an alien invasion of Earth. You control research, there's an economic element, but the most fun part is the squad-level tactical combat, with troops and weapons that get better over time and missions and enemies that get harder. It's a great game, and last time I checked it was about $5 to download on steam. If you've never tried it, give it a shot. I bet you'll love it.
Anyway, I can't claim the idea of trying X-Com in 15mm is original to me. Fellow TMPer Thorlongus posted some pics of his Critical Mass Games Snakemen for 15mm X-COM while back. I've stolen... erm... adopted some of his ideas for my own effort. (Also have some Snakemen on order from CMG. Here's what I've got so far.
I chose to use Khurasan minis for my X-Com forces. The Sepulvedan Resistance Fighters (now listed as Low/Mid-Tech Humans in Ballistic Vests)
are a pretty good match for your initial, unarmored, low tech X-COM squaddies. As tech level goes up I'll use Khurasan 15mm High-Tech Humans in Body Armour, and eventually Khurasan 15mm High-Tech Humans in Assisted Power Armour.
In X-COM you can add small tanks to your squad called Heavy Weapon Platforms (HWPs). They come in increasing tech levels, armed with either cannons or missiles, and either tracked or hover. To represent these I got some Ground Zero Games Wheeled Drones with Autocannons and some Wheeled Drones with Missiles, as well as some UAV Hover Drones armed with guns or missiles.
Here's a few pics:

Pic above shows the basic sqaddies.

Pic above is a couple of the HWPs with a Khurasan mini for scale.

Pic above shows all of the HWPs with the same mini for scale.
Squad needed a transport plane. X-COM uses a plane called a Skyranger which resembles a space shuttle. I found a Revell Snap-Tite Space Shuttle model that was pretty close to a Skyranger. I did a little converting but it could be a lot better. It's too long and too narrow, the wing should be on top, and a lot of other little things. Here's some pics.

Pic above shows the team disembarking, led by a couple of HWPs.

Different angle.

Pic above shows the whole unit deployed.
Well, there's the first of the good guys. Now for the Bad Guys.
There are a bunch of Aliens who fight X-COM. Sectoids, Mutons, Snakemen, Floaters and assorted hangers-on. These will also have to be close proxies as nobody makes them. However, there are some pretty good choices out there on the market.
Sectoids: Rebel Minis Grays. The armed ones. I got a pack of those primed and ready for painting.
Snakemen: Critical Mass Games. Mercs-Astagar Fighters.
Mutons: Rebel Minis Super Heroes. Mutons look like heavily muscled men in tights.Several of these will make nice Mutons. They're primed, too.
Chrysalids: Chrysalids are close combat support troops that work with some of the main alien types. I'm using Khurasan Minis Plutonians to represent these.
I'm also working on a small Alien Scout ship, and a 2x2 foot terrain board. The terrain board is going to be one of the farms where many of the missions occur.
Thanks for looking,
Luckyjoe