This was intended to be a post about terrain and building things in scale, then I saw the final photo and had to add it in. I chose the first two because they are excellent representations of things that could be terrain on a gaming table. The third one's just awesome.
This one could almost be called "See what's been on Brothers Brick!"
Aliens Dropship by Legohaulic.
The APC is the wrong scale in relation to the "people", but that dropship is great.
Lunar Module Eagle (Apollo 11) by hajdekr.
Just look at that!
This last one caught my eye because of the insane number of fantastic smaller mechs.
Alien Crawler by lisqr.
Dig into his Flickr stream and look at some of the other shots of this diorama. It's very much worth it.