How can I find what process is starting apps in the background?
CIJScannerRegister is of no use if you don't have an old Canon Inkjet, but Mavericks really wants to have it running all the time.

It's in /Library/Image Capture/Support/LegacyDeviceDiscoveryHelpers, but if you remove it it will appear again in the form of a "Canon Inkjet Printer Software Update" in the App Store.

The only solution I found was to delete the executable inside the app bundle so the OS thinks it's still installed but it can't run.

In Terminal:

killall CIJScannerRegister
cd /Library/Image\ Capture/Support/LegacyDeviceDiscoveryHelpers/CIJScannerRegister.app/Contents/MacOS/
sudo rm CIJScannerRegister