Another plug for YoYo games, because if you need to waste and hour (or three), this is the place. If you are into puzzles, you'll like these two I've started playing recently.
Cubic Conundrum
http://www.yoyogames.com/games/show/21452 This is an impressive little 3D game. You control a cube. The objective: you start at Point A and you need to get to Point B. But there are obstacles in the way. You clear these obstacles by "painting" the side of your cube by smooshing it against a colored block. You then take the painted side of your cube to the corresponding colored square on the floor. But there is a catch! If the painted side of your cube touches any other floor or wall, you lose the color and you have to go back and repaint your cube. Spinning tiles and color-zapping floors make this an interesting challenge.
There are over twenty levels and some are harder than others. Expect to spend some time trying to figure out the solutions! The good thing about this game, your progress will be saved if you need to leave for some reason.
You'll be coming back to this one over and over! (I will beat Level 17!)
Acidbomb
http://www.yoyogames.com/games/show/9http://www.vertigogaming.net/ (Author's website)
You are a member of the Bomb Squad. Whether a cooking stove becomes strangely explosive or international terrorists plant a thermonuclear bomb in the city, it's your job to defuse it! You defuse a bomb by uncovering pistons on a grid. How do you uncover pistons? You get two (sometimes three) light beams. One beam moves horizontally, one beam moves vertically. If you get a third beam, it moves diagonally. You move a beam of light over one row of the grid and it will tell you how many pistons are on that row. From there, it's just the process of elimination.
But watch out! Not all bombs are the same. Most bombs are of the regular explosive variety, but some bombs are unstable, meaning you can only click on a piston and marking anything else will trigger the bomb. Others are spontaneously combustible, meaning the timer will run out faster!
You will never solve the same map the same way twice. This is both a blessing and a curse. Blessing because it has excellent replay value. Curse because sometimes you have crappy piston positions and you can't uncover anything without guesswork. And when that happens, sometimes it's easier to just restart a map until you get something more favorable. But other than that, this is another challenging brain-teaser!
Have fun!