Why does "Phone booted!!!" toast message show up on start up? Recently, whenever I power on my phone, a toast appears saying "phone booted!!!". What does that mean? Is it a virus? I haven't installed any apps in a week or so.
Any application that uses RECEIVE_BOOT_COMPLETED permission can run when the phone boots (i.e. when it is first turned on), in your case to display a toast message. 

Try checking the permissions on your (recently installed) apps. Any application that uses "Start on boot" permission is a candidate for your problem. 

To further answer your question, I doubt it's a virus, especially if the app in question is installed from official sources. Most likely just a feature developer implemented to notify himself.