I'd recommend:
Handcent- it's got a bajillion options for how you want text messaging done.
Shopsavvy- it scans the barcode of products (think books or dvds), then uses your gps to locate stores that are local to you, as well as pricing them out. In some cases, it'll even indicate whether or not it's on stock.
AppBrain- it lets you peruse the market from the web, view "what's hot" in terms of downloads for the last day or week, and if you also download the "fast web installer", you can install other apps to your phone from their website.