TodaysMeet. This is a funky little resource, pulling tweets with a specified hashtag into a room, while at the same time allowing people to join the room and communicate directly in the room, but without the requirement for a Twitter account. The advantages of this situation are that your audience may not be on Twitter, but they can simply join a room you create on the fly, you prefer private discussions, and you just want to pull in stuff from Twitter that's appropriate.
Share places and events on Twitter. Nice little Google maps mashup service. It allows you to create a little map to go with your party/reception/event. Very simple, but works well. Good example of Surrey Libraries promoting an event using it. Thanks to Gary Green for the headsup on this one.
Scientific Microblog | Sciencefeed is science as it happens communicated through a microblog platform. Conceptualized and designed by scientists, it is a bridge between online scientific networking platforms, scientific databases, and the wider online science community.