Here's a list of places where you can add your Twitter account to increase your exposure and reach to get more people to follow you...

If you know of any more, please add to the list.

Official Twitter Search - See what's happening — right now.
http://search.twitter.com/

Tweetscan.com - In the innovative world of Microblogging, one site aims to organize it all. Tweet Scan searches Twitter, identi.ca and other Laconica-based sites with more being added all the time.
http://tweetscan.com/

Twitter Directory on EzineArticles - These EzineArticles Expert Authors have set up their EzineArticles accounts to automatically post their newly approved articles to their Twitter profile. Get Yourself Listed Here. Be sure to upload your author photo and bio to your Twitter account in order to build more credibility for your profile. Lastly, follow EzineArticles on Twitter.
http://twitter.ezinearticles.com/

Twubble - Twubble can help expand your Twitter bubble—it searches your friend graph and picks out people who you may like to follow. Click the button below to get started.
http://www.crazybob.org/twubble/

TwitterTroll - real-time Twitter search engine!
http://twittertroll.com/

Social News Watch - Top 237 Twitter Users Who Will Follow You Back
http://socialnewswatch.com/top-twitter-users/

Twitterholic - Our twittastic robots scan the Twitter public timeline for new twits to tweet. A few times a day, we calculate individual statistics for each twittering twit in our database.In other words.. WE"R IN UR TWTTR PAGES, READN UR STATZ
http://twitterholic.com/

Twitter Business Directory - Welcome to Twitter Business Directory, the free way to promote your business (whatever it may be) on Twitter...From mommy bloggers to marketing specialists to artists – this first of a kind directory brings information directly to your potential customers.
http://www.twitterbusinessdirectory.com/

We Follow - Directory of most popular twitters, by topic. Add yourself.
http://wefollow.com/

Twitterectory - A Directory of Twitter Profiles
http://www.twitterectory.com/listings/

Twellow - Search all people on Twellow
http://www.twellow.com/

Just Tweet It - The Tweeter Directory
Choose a category and find other Twitter users or list your name to let people find you!
http://justtweetit.com/

Twitter Fan Wiki -
http://twitter.pbwiki.com/

TweetWorks - Tweetworks is designed to make micro-blogging more useful for people and businesses. Tweetworks is based on the simple premise that people like to talk about stuff with other people. And it is the stuff (ideas, questions, politics, sports, arts and so on) that brings people together and around which they form community.
http://tweetworks.com/

Twemes - Twemes.com follows Twitter.com tweets (messages) that have embedded tags that start with a # character. These are sometimes called hashtags but we like to use the term twemes. Through the use of twemes, we can all view what people are talking about across the whole Twitter universe. In some sense, this can be thought of as an adhoc chatroom. We also pull in recent public photos from Flickr and public bookmarks from Del.icio.us.
http://twemes.com/

Tweet Party - Friend grouping and messaging for Twitter
http://tweetparty.com/

Twitter Packs - Welcome to TwitterPacks, where the community recommends fellow Twitter users by topic of interest or geographical area.
http://twitterpacks.pbwiki.com/

Twitter Groups - Business and Social Networking MasterMind Groups are now forming for all major cities in the USA and Canada! This is a great opportunity for you to network with other entreprenuers in your area to share ideas and questions about business and social networking.
http://twittgroups.com/

Twitter Passion - If you are passionate about social media, specifically Twitter, then this is the site for you! We are here to help you learn more about Twitter and how you can use it to grow your business, meet new friends, and network with like minded people
http://www.twitterpassion.com/

My Tweeple - Welcome to Twitter Link Up. Add Your Profile & Search for Others of Interest
http://www.mytweeple.com/

Twitter Link Up - Find, Follow & be Followed by the People that Count!
http://www.twitterlinkup.com/

Hashtags - Hashtags was designed to accommodate the real-time news community. We provide analytic reports and indexing features to allow users to track what's happening now. Hashtags are a community-driven convention for adding additional context and metadata to your tweets. They're like tags on Flickr, only added inline to your post. You create a hashtag simply by prefixing a word with a hash symbol: #hashtag.
http://www.hashtags.org/

Niche Directories

Learning Professionals - This Directory lists (in alphabetical order by Twitter username) learning professionals from both education and corporate training, as well as other related professionals and e-learning products and services on Twitter. If you are a learning professional who wants to connect with others via Twitter and would like to appear in the Directory, email me with the entry details you would like to have.
http://www.c4lpt.co.uk/socialmedia/edutwitter.html

GovTwit Directory - Looking for government agencies using Twitter or other social media? We want the GovTwit directory (below) to include all facets of government on Twitter: state and local, federal, contractors, reporters, academics, judicial branch and more. The list below will be a living list, and we hope to keep it updated via our own research, as well as your submissions.
http://www.bearingpoint.com/GovTwit

SocialBrandIndex - Twitter accounts are organized by category: Business, Education, Entertainment, Executives & Notable Individuals, For Consumers, Government & Politics, Housing Market, Media, Non-Profit Organizations, Service Providers, Social Properties, Technology, Travel,
http://www.socialbrandindex.com/twitter

Other Twitter Index Sites
www.registromarcastwitter.com (ES)
www.golez.net/irish-twitter-index (IRE)