Weatherbug
WAY better than the Weather Channel App. The notification plugin is nice, puts the temperature in your notification bar. Swipe the notification bar into the shade mode and it shows a bit more detail about the current weather. Now I can see the weather without unlocking my phone. The actual app is pretty nice for seeing forcast. Widgets (there's two!) are pretty nice as well.
Swift
Not the best twitter app, but I like it better than Twidroid because it has a fast, built in browser. So when people post a link or a photo in their tweet, you can see it without opening the main browser, which can take awhile to open.
ABCOrganizer Lite
This has really sped up my phone. Instead of cluttering up my 3 screens (stock home) with tons of icons, I use ABCorganizer lite to make little live folders. A Stands for Apps, B stands for Bookmarks, and C stands for Contacts. You can create live folders that are combinations of these three things. Great for organizing your apps. The less stuff on your home screen, the faster it runs. One of the best things about an ABCOrganizer live folder is that it CLOSES ITSELF after you click on the app, bookmark, or contact. No longer navigating back to the home screen to see a folder still open, waiting for you to close it.
BuddyMob
BuddyMob allows you to write one status update, and have it cross-post to twitter and facebook at the same time.
Locale
Easily the most useful app on my phone that I don't actually open.
When I go to the gym in the morning, locale turns down my notifications volume, turns down the screen brightness(gym is plenty bright), changes the background, and updates my google talk status to "@ the gym". This is based on two conditions: time (Mon-Sat between 6 and 9am) and location. I don't always go to the gym at the same time, and if I'm sick or end up not going, then locale doesn't change anything. When I leave the gym, once i'm far enough away, it returns to defaults.
When my partner, family, or friends call -- Locale turns on bluetooth so I can connect my headset.
If I'm in my apartment between 11pm and 4am, Locale gives me a moon background and turns down the ringer and notification sounds so they don't wake me up.
I'm a temp, and all my temp assignments are appointments in my google calendar. All of my temp assignments are downtown. So if I'm (1) Downtown and (2) it's during a temp assignment calendar item, the background changes to boring, the phone goes on vibrate, brightness decreases, Google Talk updates to "@ work, may be busy", and screen timeout goes 30 seconds.
There are a lot of plugins for locale. many of them free, and it seems every couple weeks there are more and more. There are ones to send SMS, sent Tweets, turn on/off auto-sync, turn on/off GPS, turn on/off spoken caller ID, make your computer wakeup, launch an app, etc, etc etc
K-9 Mail
It's an alternate email client. I use it for my second job with a theatre company. I chose K-9 because it had the option to set up a signature, and to BCC-self on my emails from my phone. They've recently added "push" so it seems like my emails come immediately, instead of when I have K-9 check, but I haven't really tested it. Great app, though.