Lets list down what kind of app, maybe we can share which one is really useful. For me so far:

Whats App - Standard
Malaysia Cinema - Show schedule for cinema in Malaysia
9Gag - Standard
BookViser - Reading e-book, got cloud support with Skydrive and Dropbox
Flashlight XT - Good flashlight app
FB - Standard
IM+ - For YM and MSN chat(works on background)
WP7 news - Latest about Windows Phone and misc
GNN(Game news Now) - Gaming news
GPSInfo - To make sure whether your GPS is working or not ;p
Handyscan - Scanning of documents
PC File Download - Transfer files between PC and phone(using wifi IP)
PC Remote - You can use your phone as remote mouse, keyboard or even remote access your whole desktop, but its a bit slow(using wifi IP)
SkyMusic - If you hate using Zune for transferring songs, you can use this. Just upload songs to your Skydrive and download to phone by using this app. Then play it offline. Just don't expect it to load on artist pics like Zune does.
Skyscanner - Flight schedule
YouRang - Create ringtone directly from Youtube
YouTube Pro - You can download youtube videos and play it offline