Taskr - You have to pay a small amount, but it's well worth it. You can automate many things about your phone. e.g. I turn on my GPS automatically whenever I open Google Maps, Latitudes, Locations. When my phone can "see" the cell tower near my place of work, it turns its volume down to a less-obnoxious level.
GTasks - Free, it helps you keep to-do lists and has a built-in widget to make it all the easier. Syncs with Google account, too.
Key Ring - Scan all those super shopper savers cards you have dangling from your keyring or crowding your wallet. It stores them and the stores can scan your phone.
Elixir - Also free, more in-depth info on your phone and the ability to store more icons on a page.
AppBrain App Market - Superior (sadly) to Google's own Market, easier to search, navigate and research apps.
Chrome to Phone - Pass links from Chrome to your phone. Requires an extension to be installed to your Chrome browser. Both free.
GroupMe - Only useful if you have other friends/contacts. Kind of like a chat room, allowing people to mass-SMS each other. If you use it through the app via your data-plan, it doesn't use up your text messages.
Quadrant Standard - If your a tech-head, this will run a benchtest on your phone and compare it to others.
Welcome to the Android!