Darkslide: I use Flickr (and FlickrExport for iPhoto), and this is a nice way to use the service. The killer feature is "Near Me," which shows you photos taken near your location. Simple but eye-opening.
MLB: Baseball scores and video highlights, was worth the $5 as it was constantly improved during the 2008 season.
NetNewsWire: RSS reader that will sync with your NewsGator subscription/read list, and so works well with the NetNewsWire for Mac.
NYTimes: even though it's seemingly constantly accessing the net when you use it, arguably a better way to read it than the web, since it's just text (and a lead photo or two).
MyWeather: location-aware weather for the US (although it will show the radar for Canada, too). Kinda pricey when I bought it, and maybe the Weather Channel app is better now, but was very useful on our summer vacation, when the weather was constantly threatening.
AOL Radio: I don't use it that often, but it works surprisingly well.
Bloom: This is the Opal generative music app. The kids love it.
Wikipanion: If you're going to be an insufferable know-it-all, you might as well get help doing so.
Stopwatch: cool big-display stopwatch. Not really a "need," but the price is right.