I've ended up keeping a bit of list of events listing websites here: https://wiki.openstreetmap.org/wiki/Mapping_Weekend_Howto#In... So I've added radius.to to that.

This is for running community events for OpenStreetMap, and so one quirky requirement is that ideally we show OpenStreetMap rather google maps (perhaps as an option). We also like to see open source options, of which there are a couple listed on there (more suggestions welcome!)

But all of these considerations are less important than the big one: Discoverability. meetup.com wins by having a list of events which is not just searchable and taggable etc, but also massive. So people just browse on there and find your event via their interests. That's a tough thing for any newcomer to compete with obviously.