Opera Web Browser
It's free, so there's no reason not to download it. I've been using it to check the NWS radar whenever there's a thunderstorm and my computer is turned off. It's much faster than the Opera browser for the DS Lite, though not quite as good as the Internet Channel on the Wii. Its biggest flaw is its lack of Flash support.
Animal Crossing Calculator
200 points. I've always thought the DS platform had the potential to do more than just games, now Nintendo has finally figured that out. This is a standard 5-function plus memory calculator that is vaugely Animal Crossing themed. It can also do (some) unit conversions, though it leaves out some obvious units like quarts or tablespoons. I also noticed it has a slight accuracy problem; if you divide 1 by 3, then multipy by 3, it gives an answer of 0.9999999999 rather than 1. Still, it's nice to have a calculator on hand when you need one, and it's only $2 (or free, since you get 1000 DSi Points when you buy a DSi)
Animal Crossing Clock
200 points. Nice try, Nintendo, but not quite. This app has the clock from Animal Crossing, superimposed on a miniturized town complete with tiny, barely recognizable City Folk characters. As the day progresses, the sky changes color and spaceships and balloons float across. You can set up to 3 alarms, using music from Animal Crossing as the alarm sound, and you can program your own "Town Tune" which sounds every hour, just like in the game. This sounds like a nice idea, but the DSi already has a clock and an alarm, so this is really just window dressing. You can have the display turn off after 5 minutes or 1 hour, which totally misses the point of having an Animal Crossing themed clock, or you can leave the display on and suck down the battery. If you're going to use this as an actual alarm clock, you'll want to leave it plugged in.
Clubhouse Games Express: Card Classics
500 points. I loved Clubhouse Games. It had tons of different card and board games and was a great way to waste time. This version has 5 of those games, Blackjack, Five Card Draw, President, Last Card and Last Card Plus (Crazy Eights). You can play against the computer or using DS Download Play, even against older DS and DS Lite units. You can also transfer one of the games to another DS for them to play against the computer. The computer is a pretty good opponent, even on Easy. Overall, this game is worth the money, unless you already have the full version. (I only bought it because I frequently carry just the DSi and a single game cart.) Japan has also seen the release of versions with board games and trump-style card games, these will likely reach the US in the coming weeks.
Dr. Mario Express
500 points. It's Nintendo's own version of Tetris. It's highly addictive and you can find that you've wasted a huge amount of time on this game. This is apparently a port of WiiWare's Dr. Mario Online Rx, only without the multiplayer. For some reason, the touch screen is not used at all, which is a bit confusing at first. But before you decide to purchase this, you should know that Brain Age Express includes "Virus Buster", a clone of Dr. Mario. If you're planning on buying that game, you might as well save the 500 points for this one.