Here's a short list of applications I'm currently using on my HTC Incredible running Android 2.1:
Handcent- better than the stock sms/mms tool and highly customizable
Evernote- I use this for note taking but its works beyond just notes; it syncs online and can upload photos, documents, etc.
LastCall- this runs in the background and is activated when your phone call is through. It will allow you to choose from a number of actions, such as converting the call to a task, calendar appointment, etc. Makes the phone much "smarter"
Seesmic- If you use Twitter, then you might like Seesmic better than the stock app. Many more features and allows for multiple accounts.
Anypost- This is a great information sharing tool that allow you to simultaneously post web articles, pictures and other items. Useful when posting something to Facebook, Twitter, Blogger, etc all at once.
While there are literally thousands of apps out there (and probably another dozen or so that I have on my phone but don't use for real estate purposes) I find I use these several times a day to get basic tasks done.