Geofencing apps?

Asking on behalf of a client, and admittedly it's not one of my strong points, so going to you experts. What are you using for geofencing capabilities? Specifically they're looking to show search/display ads for users once they're in the vicinity, but we've only looked at one tool so far and I'm sure there are more. (Not asking for testers or beta info, just hoping to gather a list of options that we can take back and explore). TIA!