While researching how Twitter was created, I ran into this really great Techcrunch article that announced the release of Twitter. In it, they compare it to Dodgeball and someone else commented about it being similar to Pluggd.
Dodgeball was a service that was founded in 2000 by an Alex Rainert. It let you text your location and it would let all of your friend and friends of friends know that you were nearby. It was bought by Google in 2005 when they were trying to move into the mobile space and was shut down in 2009 the same year that Foursquare was released.
Pluggd was not really a competitor to Twitter and I think the only reason why the commenter mentioned them was because they were also a podcasting company like Odeo was. Odeo was what Twitter was before it became Twitter. Like the pivot that Odeo made to become Twitter after Apple moved into the podcasting space in 2006, it appears Pluggd moved into a very specific niche while maintaining their podcasting platform. They were providing speech recognition software for videos to recognize topics for marketing purposes. It appears to be shut down and they are now a video delivery platform called Delve Networks.