1) FBReader, without which I would do much less reading and be much more bored waiting in lines. I'm currently reading my way through the Project Gutenberg SciFi section.
2) Web browser / feed reader (I use Google web-based reader, not their app-reader).
3) Calendar (without reminders, I miss meetings).
4) KeePassDroid.
5) Camera / photo gallery.
6) Contacts / Phone.
7) email, but mainly when I'm on the road and don't have easy access to a laptop + network.
8) Dropbox - probably should be higher in the list, but it quietly does its magic in the background so I forget about it.
Five of the eight came with the phone. Of the three add-on apps, I would sorely miss FBReader. I survived without KeePassDroid for a long time, but it is awfully handy to have sensitive information at my fingertips. Dropbox is very convenient, but only a convenience.
I have 29 add-on apps total, but I use most of them only occasionally.
Opera Mini
Flixster
Google apps (voice,Gmail,maps, navigation,Docs,search)
Weatherbug
Github Viewer
SoundHound
SongBird
myxer
I spend most of my time in Opera Mini browser.
Byline
Tweetbot
news:yc
Instapaper
Dialvetica
Calvetica
Highlight
Thoughtback
Soulver
OmniFocus
Simplenote
iPod
Safari
Carcassonne