twitlonger - if you need to write a tweet over 140 characters, then write it at twitlonger and it will be posted to your stream with a link to twitlonger to read the rest…also see tinypaste, Twitwall, and Twitblogs.
Tweetizen - create a Twitter group. You can filter tweets by keywords or hashtags, and you can limit the stream up to 10 sources (tweeps). Plus these groups are public. I made a quick group from some tweeps in my network who are book authors. This is similar to Filttr (which is not public), but there is no limit on sources. See more on Groups on Twitter.
FriendFilter - similar to Topify, Twimailer, Nutshellmail in that when you get an email of a new tweep following you, it gives you a heap of information. Some of which I like is that is displays mutual friends (friends we both follow, and friends who follow both of you). It also acts as an address book where you can click on a friend and see their latest tweets and profile. It displays your DM’s and replies, a snapshot of your stream and friend stream, and you can post a tweet. [via b]
Twibes - join an open group…streams latest tweets of group, has RSS feed, you can reply, and keeps you updated on new members