Phylo is a browser-based colour/block matching game available to play here. Like the famous FoldIt, its aim is to 'recycle' the power of thousands of bored casual gamers to further a scientific objective, but while FoldIt concerned itself with protein sequencing (or manipulation or whatever the term is; I did engineering, damnit, not this namby-pamby squishy nonsense), Phylo has you sequencing (or matching, etc. etc.) genomes. If you're interested in the back-story Rock, Paper Shotgun has an interview with one of the scientists involved, but I'm concerning myself more with the actual gameplay.
The basic mechanics are extremely simple. You are given two rows of coloured blocks, and have to match them up with as few mis-matched colours or gaps in the sequence as possible. You can slide the blocks left and right (not up and down), but they can't change order and so shunt each other aside. As the game progresses more and more rows are added, and so it gets more and more challenging.
I found the hardest part of the game to be understanding the scoring. You get one point for every correct match; you lose one point for every mis-match; the first gap in a sequence loses you five points, with every subsequent gap losing you one more point; and you get more points for matching closely related species than unrelated ones. This last part is especially relevant once more rows are added, and prioritising which rows to match becomes important. The scoring is explained briefly in the tutorial, but you can't look it up while playing (thus violating one of the cardinal rules of tutorial design: that all information learned therein should be available at all times), and I couldn't find any explanation as to exactly how much more valuable closely related species matches were, which makes it much harder to work out what you should be doing.
It also took me an embarrassingly long time to understand how gaps work. It's not just about breaking a sequence: if you have one sequence longer than the other, then any space between the ends of the two rows also counts as a gap. So in the example above, the bottom row (the bat) has no gaps, but the top row (the cow) has two: one on the left (above the green bat square), and one on the right (from the orange to the green bat squares). Maybe I'm just really thick, but once I got onto four or more rows the scoring ended up feeling random and confusing, which made for a rather unenjoyable experience.
It wouldn't be so bad if the scores were displayed in real time, giving you some feedback, but instead the score is calculated every five seconds with a little red timer bar and an irritating blip noise, which means that you either have to wait or manually click a button to see if your latest move has improved your score or ruined it. This disconnects the player from any sense of feedback and makes understanding the already complicated system ten times harder. In, say, Bejewelled, this wouldn't be such a problem, but here the score is everything: your only objective is to increase it. I really have no idea why a timed update was chosen over real-time. Maybe it's an optimisation decision, but I refuse to believe that it could really save any meaningful amount of processing over simply having it update each time the player makes a move.
The other big annoyance is that these challenges are timed, which means that a good proportion of players fail to complete the challenge at all, let alone in an optimal fashion. The difficulty curve is what I'd describe as 'exponential with wobbly bits': the basic levels with three rows are absurdly easy; four rows can be pretty tricky, and anything more than that has a good chance of being fiendishly hard, though occasionally you'll come across an eight-row puzzle that can be solved simply by stacking all the blocks to the left. This is exacerbated by the time limit, which, as far as I can tell, is the same for all difficulty levels: I started off doing some basic three-row puzzles, got bored, moved to the next level and ran out of time.
Having said that, there are also a lot of problems that are definitely due to bad design: the intermittent score updating; the way that useful stats are only shown when mousing over a certain button; or that there's no way of abandoning a level once started without logging out. And why the relative values of related species matches couldn't be explained I have no idea.
I'm probably being too harsh. It's a free game for a good cause, and even without the feel-good factor of progressing scientific knowledge and being part of a big, happy crowdsourcing experiment it's more challenging and worthwhile than Bejewelled (though it's definitely more frustrating). I'd recommend that you go and give it whirl, but I wouldn't blame you if you don't play for very long.