参った (まいった) means something like "I lose" or "I've been beaten." If you're thinking in terms of current slang, it might be close to how the young folks these days use "fail." Interestingly, 「参る」 is also 謙譲語 (humble form, usually used along with keigo) for "to go" or "to come," but it also can be used to mean " to be overwhelmed," "to be defeated," etc.
I think I've also heard this one used in my Japanese lesson DS game, where the girl will say 「まいったなぁ」sometimes if you get an answer wrong. I'd be willing to bet that it's used fairly often in anime, as well. Below are some more examples of natural usage:
First off, here we have a blog entry posted by someone whose computer appears to have crapped out on him. He tried restarting it several times, but to no avail. If the system restore doesn't work, he is completely out of luck. Fail.
Next up, a commercial for Boss coffee that couldn't be embedded here, so all I have for you is this lousy link. Basically you have a salaryman who looks to be low on sleep (and the lyrics of the song in the background indicate so), but that won't discourage him from getting to work like he does everyday. Hey, when life is a fail, at least there is Boss coffee.
Last, here's one more video for you. Some guy opens up his bento to find that his egg...well, just play it and see. Not a big deal, but still an egg fail of sorts.
Anyone else have any good examples of 参った?