I agree, Most Mac app’s don’t have un-installers, and most users drag them to the trash to remove them. That works for most apps but can leave various “system files” on the hard drive. The uninstall process on a Mac is usually not critical like on a windows machine were thinks like entries in the windows registry can harm the performance.
I use an open source app called AppTrap; it runs a demon in the background that monitors anytime an app gets deleted. Anytime an app gets moved to the trash you’re asked if you want to move the associated preference files as well. You can expand the window to see a list of what is going to delete:
The thing I like about this product is it doesn’t "automatic delete anything. You can decide if you want to delete it or not. BEWARE: THIS WINDOW POPS UP EVEN IF THE APPLICATION IS UPDATING ITSELF. So you may not want to leave this running all the time, but toggle it on when you are cleaning up old apps. You can control AppTrap from within system preferences:
This is a real-time app only. It’s not going to go through your system and try to clean up old apps that have already been deleted. It also won’t get everything from more invasive applications.