http://conceptvehicles.blogspot.com/

http://concepttanks.blogspot.com/

http://conceptships.blogspot.com/

it takes a bit of digging sometimes, but there are some truely inspired pieces of art in here

Totally Awesome... Thanks...

Frankly, those are cool sites (and I've browsed through them before seeing this topic), but just reading about the x-planes and experimental vehicle models does it for me. RL is much weirder than most things imagined :3

Very cool. Thanks for sharing!

Wait, there's http://conceptrobots.blogspot.com/ too!

And, while this may be less useful for some: http://www.creaturespot.com/

But when you just want to go beyond books to give your players extra scare - browse those. Just scroll past the dressed mice in the beginning...

Some other inspirational sources

Evironments : http://www.conceptart.org/forums/showthread.php?t=56501

Industrial/Tech: http://www.conceptart.org/forums/showthread.php?t=77149

Creatures: http://www.conceptart.org/forums/showthread.php?t=37514

I love the conceptart website and just put direct links to the compiled winners thread (they have periodic contests in certain categories) but there is plenty of awesome art that is not in these particular threads. Generally pretty awesome to look through.