While having to troubleshoot some custom button links that included hardcoded object and record IDs from a different org, I stumbled upon this cool list. Thanks to Daniel Ballinger for making this table that lists Salesforce object key prefixes. 
In my case, I had an object ID for an org that I no longer had access to. I needed to ascertain if the object was a standard or custom one. If the prefix isn’t in this list, I can deduce that it’s a custom object and move on with my troubleshooting.