Prey

Prey did portals before Portal.  It didn't do them as well, but it's still a very creative, enjoyable shooter.  Earlier this year, it was offered on Steam for the cheap price of five bucks, and I couldn't pass that up.  It's not a great game, but it's fun, and easily worth that price at least.  There aren't that many weapons in the game, but all the firearms are pretty interesting visually, being made of either advanced technology or small aliens, and have multiple uses.  Most enemies are pretty dumb and don't make for particularly entertaining confrontations, but Human Head Studios throws in enough gimmicks, from ghost children to gravity-defying walkways, to keep it fresh.  You can also use your spirit powers (Native Americans are magic!) to fight in other ways and solve some environmental puzzles.  Trudging around and shooting up aliens could have gotten boring, but all the disorientating, unique environments prevent repetition from getting too grating.  The puzzles get too simple once you figure out the different tricks they have, but that only really manifested as a flaw near the end, when what should have been a set-up for a gigantic, brain-bending final challenge ended up being tragically easy.

Visually, the game is pretty impressive even though it came out a couple years ago.  This comes mostly from the effects and architecture of the giant ship you spend most of the game in, which is really inventive and varied, and best when it opens up to reveal grand, far-off vistas conveying the scale of it all.  The character models are less impressive.  I'm not sure if it's something with the perspective, but their heads seem too big and they animate a little awkwardly.  The sounds of the alien setting are good and some of the music choices are really nice, especially the best use of "(Don't Fear) The Reaper" I've seen, but the voice acting is uneven.  Some are fine, including Art Bell playing himself as people call in to describe the alien invasion going on, but others, including the main character unfortunately, are just clumsy.  The story is decent, and does an adequate job of unfolding naturally as most modern shooters try.  It ends on a cliffhanger, which I'm semi-interested in seeing the supposed sequel resolve.