How to label a sponsor/ad

Say you have an ad or sponsor you want to put on the sidebar of a site. I'm curious if anyone has some experience or pointers on how to label it.

You could just put a little "AD" or "Sponsor" marker, but especially for indie sites that feels a little less personal than something like "Brought to you by", "Help support the site" or even more specific like "Server costs supported by".

This is obviously a good case for A/B testing but I'm curious if anyone else has experience.