As described by the developers,

Today, commenting systems are implemented by individual web sites and have no ability to talk to each other. Users are typically anonymous or must create accounts on each of these sites. Also, comments are often moderated, preventing people from truly being able to express themselves when their opinions are in contrast with those of the web site owner.

The video below describes how AddATweet functions,

Welcome to addatweet! from addatweet on Vimeo.

Post from Twitdom - AddATweet

Here is a video demonstrating the usage.

twshot demo from angelabs on Vimeo.

Post from Twitdom - twshot

Glue works automatically as you browse popular sites about books, music, movies, wines, restaurants, gadgets, stocks, actors, tv shows and other everyday things around the web. The Glue bar appears right on your current page to show you friends who looked at the same things and what they thought!

Post from Twitdom - Glue

Features:

• View your Facebook and Twitter updates, right on Friendbar
• Get a stream of your friends Facebook photos right on Friendbar
• Roll over the photos to view an enlarged version
• Click on the enlarged photo (or album name) to visit the photo or albums web page
• Get an alert when a new message arrives
• Send Twits and Facebook updates from Friendbar (separately or both together)
• Automatically create a tiny URL for the web page you are viewing
• Click on links in text updates to open the link in a new tab
• Toggle Text updates and Photos on and off

Post from Twitdom - Friendbar

It is available under various pricing options, starting from a completely free plan going all the way up to $99 for a Pro account.

Silentale is available as a firefox add-on, a search plugin. An iPhone app and Outlook plugin are being launched shortly.

Post from Twitdom - Silentale

OneRiot aims to differentiate itself from the crowd by ‘prioritizing information based on its current popularity with the community’. This is an attempt to make OneRiot’s search results relevant, and fresh.

OneRiot also provides a Firefox Search toolbar.

Post from Twitdom - OneRiot

Post from Twitdom - Glydo

You can stumble based on trending tweets or based on a search criteria you provide.

StumbleTweets also provides a Firefox toolbar that you can use to stumble!

Post from Twitdom - StumbleTweets

Select the text on the web pages and tweet it to your friends Add link to the source with Chilp. A version for Internet Explorer is also available.

Post from Twitdom - CloudBerry TweetFox

See the image below for an example.

Post from Twitdom - Twitter Search and Tools