Plazes allows you to add your physical presence to the web - if you are at the office, at your local pub or coffee shop, you can tell Plazes to locate you. You location appears on a Google map. Other Plazes users can see you, and you can have friend lists - basic social networking stuff.
Where it gets interesting, is that you can identify Plazes - locations in the real world - and add information about wifi connectivity and URLs. Ac icon is also associated with the Plaze - showing it to be a meeting place, hotel, airport, or other physical world Plaze. But why can't comments about the location be added? Now that would be useful.
I really think there is potential here. Imagine hosting a meetup and having it appear on Plazes so people could easily find you. If a significant number of people at a conference used Plazes you could see where the after-hours parties are, or where groups are meeting and, if comments were enables, what they were discussing, in case you wanted to join in.
Plazes is one of those ideas that is almost there. It just needs a nudge to make it really useful.
