Maker Swipe File - 100+ high-performing Tweets to help you grow your audience
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Narrow - Narrow helps you build a targeted audience and increase your influence on twitter.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Tweetflick - Save, organize & find Tweets to get the most out of Twitter
Bigtent - BigTent is a web-based organization and communication portal for groups.