Places Directory is essential
Quickpedia is very useful
NewsRob is useful to pull all the RSS feeds to one place (Google Reader account required)
Linda Manger is essential when wanting to browse files on your phone.
GDocs when wanting to make quick notes, or documents, lists or whatever.
Those are the 5 perhaps most useful to me although i use social networking, and games more often.
other useful ones (not default apps that came with the phone) are google shopper, shopsavvy, dictionary.com, food finder, howcast, mybackup, shazam, any news/weather one, sportstap, soccer live scores,