Matthew McGiffen goes searching:
The script uses the undocumented sp_MSforeachdb internal stored procedure to iterate through all the databases (excluding the system ones) and store the results of a standard unused indexes script into a temp table. The table can then be queried to analyse the results.
This script comes with a caveat, that the sp_MSforeachdb procedure can be unreliable and on occasion skip databases so you should check that the results include all the databases you are interested in. A simple count of distinct database names, and comparing that against the number of databases on your instance should be sufficient.
That caveat aside (and if you are concerned about it, check out Aaron Bertrand’s sp_foreachdb), click through for the script. I do like that this limits you to just zero-read indexes, as even write-heavy indexes may still be useful.