In practice, however, it's easier said than done. The site relies not on info from Blizzard's servers, but from visitors to the site -- just click a realm name and you can input what quests are available. But anyone can do that, and there's no accounting for whether the quests are real or not (I accidentally entered some probably false information). I think the quests do reset every day, so at least if someone enters the wrong quest it only stays up for less than 24 hours, but still, you have to trust that what's up there is right.
In fact, I'm surprised Blizzard hasn't created something like this to check every day -- they've got a calendar of BG weekends, so it's natural that they'd somehow let us check what quests were available out of game, and maybe even be able to plan instance runs ahead of time. At any rate, until then, WoWDailies will have to do. If you're lucky enough to have someone updating your realm correctly and on time, it should be a good resource.