Here's a list of twitter tools and services others are using:

Twiveaway - Allows you to build your list by running your own giveaway contest
Tweetdeck - all in one twitter desktop application
Twuffer - for scheduled tweets
Twitter Counter - creates a follower count icon for your blog
Twitter Tools Plugin - creates a complete integration between your WordPress blog and your Twitter account
Lab Tweets - A software program created by a fellow warrior that does lots of cool things(
Hummingbird - All in one marketing tool
Hootsuite - For engagement and interaction
Tweet Adder - Get more targeted twitter followers and automates posts
Local Tweeps - The zipcode level twitter directory
Tweep Search - Lets you keyword target peoples bios
TweetMixx - More links, less clutter
Twhirl - a desktop client for social software such as Twitter, Friendfeed, identi.ca, or seesmic
Twuugle - great for targeted Niche Research and all the results are from twitter.
Big Tweet - Share links with your followers without leaving the current web page
TwAitter - Automation, rss integretion, timeline management and other cool features
Seesmic - A desktop client to manage your lifestream from Facebook & multiple Twitter accounts.