WarioWare: Snapped! is a fun little game, if you're in the right conditions to play it, that is. It's really short, but it can provide some fun times, especially if you're showing it to other people. The 500 DSi Points price tag is a bit high for what you get, but it's a neat little game to have nonetheless.
Mario vs. Donkey Kong: Minis March Again! is a fantastic game, capable of providing hours of fun. It has enough content for it to be a normal retail game, so at 800 DSi Points, it's a steal. Not only does it have tons of puzzling levels to keep you occupied for a long time, but it also has a level editor in which you can create your own levels however you want, and you can even share them online and download other people's created levels. If there's any DSiWare game that's a must have, this is it.
Art Style: PiCTOBiTS is another great addition to the Art Style series, and old school gamers in particular should love this game. The object of the game is to line falling squares of the same color in order to piece together an 8-bit NES sprite. And then there's the music, which is perhaps the best part of the game. It's all classic NES remixes, performed by the famous chiptunes band YMCK. At 500 DSi Points, it's definitely worth it.
Mario Clock may sound like a silly little app, but it's very useful. In fact, I wake up to it every morning! You can set an alarm to play one of three Super Mario Bros. tunes (the main theme, the invincible theme, and the underwater theme), or a recording of your choice. Also, if you just want to kill some time, you can control Mario as he runs through levels on the top screen and collect coins. A fun little bonus. Another great app is Mario Calculator. Having a calculator on your DSi can come in handy, and it even has a unit converter, with which you can convert such units as length, area, weight, speed, volume, temperature, time, and even age. You can also control Mario on the top screen in this app, too. If you prefer Animal Crossing to Mario, then you may want to download the Animal Crossing Calculator instead. It functions identical to the Mario Calculator, except with an Animal Crossing theme. There's also an Animal Crossing Clock, which functions exactly like Mario Clock, but I'd recommend Mario Clock even if you prefer Animal Crossing, because the choice of sounds in Animal Crossing Clock aren't very suitable for an alarm. Then there's also Photo Clock, which is the same as Mario Clock and Animal Crossing Clock, except without the video game themes, and Photo Clock also lets you display your own photos. All five of these apps are priced at 200 DSi Points (and Photo Clock is included free with any DSi XL), and what you get for it can be very useful, so I highly recommend downloading any of the Clock and/or Calculator apps.
Mixed Messages is a great game when you have lots of people around, and it can provide hilarious moments and lots of fun. Basically, it's like the "telephone" game that you played in elementary school, mixed with Pictionary. You're given a sentence (or you have to write your own sentence), and you have to draw a picture that describes that sentence (or pass it to the next person, if you wrote the sentence yourself). Then, you pass the DSi to the next person, and they have to guess what the original sentence was from your picture. Then the DSi is passed around again, and the next person has to draw a picture that describes the sentence that the last person wrote. This continues until the DSi has been passed to everyone, and then you get to review everything that was drawn and written. It's hilariously fun, and is guaranteed to make everyone laugh. It's 500 DSi Points to download, so if you have friends over often, Mixed Messages is an ideal purchase.