Tommi posted this already last week, but still think it valuable to share:
ESSENTIAL ADD-ONS for S60:
- Nokia add-on stuff
- Smart2go a.k.a. Nokia Maps
- Nokia Podcasting
- Nokia Mobile Search (get the latest version)
- WidSets
- Barcode Reader (not that useful yet, but loads of potential)
- Download! Client a.k.a. Nokia Catalogs - 3rd party consumer apps
- Y-Browser
- Handy Weather
- AutoLock
- DivX player
- ShoZu
- Calcium - Internet stuff
- Google Reader (using desktop version and mobile version simultaneously)
- Google Maps and GMail mobile
- Yahoo! Go
- Gizmo project
- Fring
- FON - Enterprise software
- Nokia Intellisync Mobile Suite 8.0, especially Nokia Intellisync Wireless Email - Open source apps
- Internet Radio
- OggPlay - Applications for industry insiders
- Screenshot 2
- Best Taskman
Other good shortlists:
AAS: S60 3rd Edition Essentials [updated]
OSNews: Top Ten Freeware Applications for Symbian S60 3rdEdition
3-Lib: S60 3rd Edition freeware
Impressive list with some essential and some not so essential add-ons, it all depends what kind of user you are of course!