Specialized volunteering

Volunteering time instead of money and volunteering time in a way that you would normally be paid for (and ideally provides a needed skill).

Providing pro-bono law services to a non-profit. Building an app for a charity.

Slightly less specialized would be donating time in a way that many others could also provide. Of course, this does not assess the quality of the volunteering alone, since scarce supply of volunteers may justify a non-specialized volunteering. Similar to Can anyone else do what you do better than you?