I've been disappointed by the results that can be found on google for blogs and articles about great twitter tools, so I decided to post my own discoveries.  Many articles contain useless twitter tools, and only mention one or two of the best tools available.  Here's a comprehensive list:

WeFollow - List yourself and find relevant people in your area or with similar interests to follow and connect with. 

MrTweet - helps you find people you might want to follow.  Good to at least list yourself / sign up with this service.

FriendorFollow - as your twitter grows, it can be more difficult to find who's following you.  This service shows a list of who's not following you and can help you even out your ratio.

Hootsuite - set your twitter (and facebook page) to post tweets at a later time.  ie - schedule some great quotes and thoughts to go out over the next week or month (one a day, or however frequently you choose to post them).

SocialOomph.com - set up personalized automatic DM's, set your account to automatically follow those who find you or unfollow those who "drop out on you" and much more.

Tweetie - best all around twitter app for the iPhone or iPod Touch. cost $3

TinyURL - best service to shorten a link for your tweet.  Looks the most professional.  Easy to do.

TwitPic - best service to add a picture to your tweet.  The other services look bad.

Twitterholic - shows you your ranking for your area.

Twitterfeed - best tool to automatically feed any of your new blogs to your twitter profile.  Very customizable - not like every other service that does this.