1. Tweet a file : Tweetafile brings an important email functionality to the simple world of Twitter – attachments. Through Tweetafile, Twitter users can send any files (images, PDFs, Docs, videos… whatever) to all of their followers or via direct message to individuals.
2. Remember The Milk : If you use Remember The Milk to manage your tasks, you can do so via twitter using this app. Add Remember The Milk as your twitter friend and simply DM to add or interact with your tasks.
3. Twittonary : If you are new to twitter, some of the words twitter users use can be quite confusing. Get help from a twitter dictionary and learn the “twitter lingo.”
4. Twonvert : 140 character limitations at times can really hold you back. This app converts your text into SMS shorthand language and allows you to say more with less characters.
5. Twist : Allows you to see “hot trends” on twitter. You can also use the search box to see what people are saying about a particular thing and how much buzz it is creating.
6. WhosTalkin : Although this app isn’t specifically for twitter, it is a very useful twitter tool. It allows you to search conversations that you are interested in. This helps you interact with like minded people or the whatever subjects interest you at that point in time.
7. TweetWhatYouSpend : Manage your spending online using twitter. Create an account and use Twitter’s “DM” feature to send tweets. You can export a spending journal later as an Excel or CSV file.
8. TwitterJobSearch : This service allows you to find job postings on twitter. If you are looking for a “web design” gig, simply enter web design to see what jobs are available related to web design.
9. IsTwitterDown? : If you are a heavy twitter user this service might be quite useful. It allows you to find out if twitter is down for everyone else or just for you. The page displays a simple answer “Yes” or “No.”
10. Twiddeo : Upload videos to Twiddeo from your camera phone while on the go. The service then generates a link and posts it to twitter for your friends and followers to see.
11. TwitterCal : This tool connects your twitter account to your Google calendar. You can add events to Google calendar from twitter by sending direct messages to gcal.
12. TwitterFeed : Twitterfeed helps you send your Blog RSS feed or atom to twitter and also lets you track the number of clicks your feeds are generating.
13. PollDaddy : Create polls and post it directly on twitter. This can be extremely valuable when you are researching a subject or need user feedback.
14. LinkBunch : Due to character constraints, it can be quite hard to send more than one link in a tweet. If you need to send multiple links in one tweet (example : a post that has part1,2,3…), this service can be useful. It collects all links and gives you one shortened link that you can post on twitter.
15. Tweetdeck : One of the best desktop twitter apps out there. Tweetdeck helps you sort your friends and followers in groups and also helps you manage replies, DMs and search results.

Twitter Clients for the iPhone

Hahlo – Nice looking Twitter client with simple navigational menu system.

iTwtr – Download tweets to iPhone for viewing even when not connected to wifi, 3G or even Edge.

PocketTweets – Slick client for the iPhone

Twitteriffic – One of the most popular clients for Twitter on the iPhone.

TwitterFon – My personal favorite Twitter client for the iPhone because of simple navigation buttons that allow you to jump from one area to another.

Niche Twitter Apps for the iPhone

AirMe - Send photos from your iPhone to Twitter with this free app which includes a Twitter client.

iMapMyRun – Lots of features for runners including Twitter access.

Jott – Record voice messages from your mobile device and post links to it on Twitter.

Locly – Location tracker with Twitter integration that shows tweets in your area.

Loopt – Another location tracker with Twitter connection but also other services like Yelp.

Twinkle – Location based Twitter app that utilizes the GPS feature.

Twitfire – Upload photos to twitter and more.

Twitterlator – Manage multiple twitter accounts.

Twittervision – A world view of tweets from around the globe in real-time.

Twootball – Track all football scores and teams via this Twitter app.

T-Mobile G1

TwidRoid – Probably the most popular full-featured Twitter client for the Android G1 now with GPS support.

Twitli – Simple client for Android devices that translates languages.

TwitterDroid – Not to be confused with TwidRoid, TwitterDroid was created as a clone of the popular Twitterific client. Not as easy to install as all of the other apps because it requires some manual sets.

BlackBerry

Blackbird – Twitter client for the BlackBerry. Here’s the over-the-air installation file.

TinyTwitter – Actually works not just on BlackBerries but also Windows Mobile devices and any Java-enabled mobile device.

TwitterBerry – The most popular Twitter client for CrackBerries. I used this one for a while on different BlackBerry models.

Windows Mobile

ceTwit – Windows Mobile client for Twitter written in c# and .net framework.

GPSTwit – Adds location tracking to Twitter on Windows Mobile devices (iPhone too)

PocketTwit – Free client (donations accepted) that works with Twitter and Identi.ca accounts.

Twitter2Go – Very basic Twitter app with limited features.

TwitToday – Small app for Windows Mobile devices with standard Twitter features.

Twitter Answers – The Twitter version of Yahoo answers. Ask and answer questions with this application.

Quakk – Open source Twitter client, which means not as quick and easy to install and update.

Smartphones

twitxr – Upload photos to Twitter (Native clients available for iPhone, WinMo, Nokia/Motorolla)

Moblf - runs on many platforms and connects to Twitter and Friendfeed

MobyPicture – Connects to Twitter, Flickr and other networks on different mobile platforms.

TreoTwit – Twitter client for the Treo and its Palm OS.

Twibble – Twitter for the Nokia N95 with GPS support.

Non-Smartphones

JibJib – Tiny Twitter client for non-smartphones for basic functions

jTwitter – Java client for Twitter that will work on many different mobile devices including non-smartphones

psyhzzz – Extremely tiny twitter updater that works on most non-smartphones

TinyTwitter – Works on many Java enabled devices as well as Windows Mobile phones.

Twim – Java client that will work on most non-smartphones.