I'm using
3gwatchdog to monster data usage
Handcent – better sms/mms client
Navigator – Satnav part of Google maps and its great
The various bar code scanners are a fun gimmick
Google sky
As for app killers, Google "why I don't need task killer in Android" – apparently its much more economical to leave things in the background rather than force them to close.
Also there seems to be a bug in the calendar app on some handsets that stops the device going to sleep – hence severely affecting battery life. Easily fixed by force stopping it in the application manager (settings, manage apps, calender). Again, Google it for more info.
Enjoy – I love mine 🙂