Here is a quick roundup of my top 5 android apps. I consider these a must have on any android handset.

WaveSecure
WaveSecure is the must have security app on your phone. It can help track down lost or stolen phones, remotely track the location of the phone/sim information, and even remotely lock or wipe the phone is needed. In addition, you can use it to backup/restore data on your phone. Finally, if your phone is stolen and a new sim card in inserted, WaveSecure will lock down the phone unless the secure code is entered.
WaveSecure is currently free for Android but costs $19.90 per year subscription on other platforms.

Contact Owner
You can never be too paranoid. Another security app that displays your (or anyone else’s ) contact information on the “lock screen” of your Android device, so that if you happen to lose it the finder will know how to contact you.

Advanced Task Manager
While there are other free task managers, this app is a step above and beyond. The user interface is well thought out, clean and professional. You can view process/system level information, memory footprints of currently running apps and if you have root access, you can terminate running apps. You can also white-list apps that should not be terminated.
Currently sells for 99c.

Google Voice
If you make plenty of long distance/international calling, this is a must have app. No longer do you need to call a central line, remember passcodes and then dial the international number. Just setup international dialing through voice, and dial away just like you would any local contact. International rates are very reasonable and sound quality is impeccable. In addition, messaging to and from voice is free and no text messaging charges apply. Check out the site for all the other goodies that generally apply to Google Voice and aren’t specific to Android.

Wifi tether for Root Users
This doesn’t apply to the community as a whole because you need to be running a custom ROM with root access. This app created a wireless access point to which you can connect your laptop to and then tether connections through your phone’s data connection. It couldn’t be easier to setup, supports encryption and in general works great.

There are many other apps that came close but didn’t make the top 5 cut. Some of them include Gesture Search, Juice Defender, Meebo IM, Shazam and Sipdroid.