I apologise in advance for what I'm about to do to your productivity. Hex FRVR is a free browser-based tile-placement game that behaves like hexagonal Tetris. You use a limited selection of preset shapes to fill up the board. Link two edges and the cells along the connecting line vanish, giving you lots of points. The game ends when you can't place any more shapes.
It's so quick and simple that I can't stop playing it. The shapes are designed to create awkward single-hex gaps in your placement, which makes the single-cell shape quite precious (I'm saving two of them in the screenshot above). Naturally, you want to build the biggest chain you can by setting up lots of almost-complete lines and then clearing them with a green line. Like this:
Unlike Tetris, there's no time pressure, so when I screw up I know it's due to poor planning rather than panic. The way lines disappear one-by-one, chaining into a combo, is very satisfying, and the game calls you beautiful when you clear a big chain. Aw.
My highest score is 4080, which I'm hoping to smash during the lunch break. Thanks to Tom F for the recommendation.