Here are some useful tools and services for Twitter users.

Windows Client:

• Twitteroo is a twitter client for windows that lets you update your Twitter status from your Desktop.

Mac Clients:

• Twitterific is a lightweight Mac client by Icon Factory.
• TwitterPost for the Mac allows you to easily post to and read from your Twitter feed. It also integrates with your other Mac apps like iTunes and iChat.

Firefox Addons:

• Tweetbar is a cool Twitter sidebar for Firefox and Flock.
• Twitter Firefox Search Plugin allows you to quickly post statuses (or “tweets”) to your Twitter account from the Firefox search bar.

Wordpress Plugins:

• TwitterTools Wordpress Plugin creates an integration between your WordPress blog and your Twitter account.
• Twitt This adds button to your WordPress blog posts which let visitors send the post url to twitter account.
• Simple Twitter allows Wordpress blog owners to add Twitter messages to their templates.
• TwitTwoo Wordpress Plugin allows you to display and edit your twitter status directly from the sidebar of your blog.

Twitter on Cellphones:

• Jargong is a cellphone social networking application with inbuilt Twitter support.
• Twapper lets you keep up with twitter on your mobile phone brower. It also has support for Flickr images, enabling photo sharing over Twitter.
• Twitter Flashlite Client for Cellphones. FlashLite 2.x application to update Twitter status from cellphone.
• Twitter Theme for Sony Ericsson W850.

Developer Resources & Tutorials:

• Twitter API: Twitter API.
• Twitter Fan Wiki: Place to post cool ideas, uses and feature requests for Twitter.
• Twitter PHP Script: Simple Script to post to message to your Twitter account using PHP and Curl.
• Scripting Twitter with Curl
• RSS to Twitter. Php script by PaulStamatiou.
• Add Twitter to your blog, without it hanging your site
• Posting Twitter Updates via Curl. Tutorial on posting an update to twitter using just curl and nothing else.

Web Services:

• Twitter Badges: Javascript badges to show twitter messages on your website.
• Twitter Buzz shows you what people using Twitter are linking to.
• Twitt This help you send webpage urls and description to Twitter.
• Twitterholic: A site that ranks the top 100 Twits based on friends or followers.
• Autotwit is a very useful web service that let you preset twitter messages that you want to send at a later time. AutoTwit doesn’t require any separate registration; you can use the same Twitter username and password.
• TwitterMaps is a mashup that allows you to send posts about your current location. According to the FAQ, just include the correct command and your current location will be viewable on the Twittermap
• TwitterVision uses Google Maps to display what Twitter users are up to and where they’re Twitting from.
• TwitterSearch allows you to search through twitter posts.
• Swotter is reading a book via Twitter to the world. You can listen to Swotter via the web, via Instant Messenger, or via SMS messenger on your mobile phone.

If you know any other interesting Twitter tool, service or mashup. Please post in the comments.