Josep brought the Zkout beta to my attention and I thought the service warranted a mention. Zkout is essentially a Twitter like website that lets you send out messages based on your location. Kind of reminds me of a Facebook status along with Twitter style messages.
From what I can tell Loopt and Where seem to be already doing the same service model so I am not sure exactly where Zkout fits in. On top of that SpotJots already has a BlackBerry compatible location based networking application. I guess it is worth keeping an eye on Zkout. They seem to be in beta since May so maybe they are still working on adding differentiating features.