Twhirl – Twhirl is a popular desktop client based on Adobe AIR so it requires the AIR download. You’ll find AIR useful for other apps also. Twhirl has a lot of cool features including multiple Twitter accounts, connect to Friendfeed, post image to Twitpic, post to Pownce and Jaiku simultaneously, and search using Tweetscan. I use it on my Windows Vista sidebar along with . . .

AlertThingy! provides a connection to twitter, friendfeed, flickr, tinyurl and allows cross commenting. It’s better than it sounds and is used as a twitter replacement by many who want to make more sense out of twitter and narrow down their focus on specific people and topics.

TwitterCal – This service allows you to post events from your Google calendar to your Twitter accounts.

Twitter Digest – This tool allows you to subscribe to a message stream from a Twitter user of a group of users on the web or through an atom feed.

Summize – A Twitter search engine, contender for the top app in this category.

Flaptor Twitter Search – A simple Twitter search engine which generates an rss feed. Also provides trends, comparisons.

del.icio.us twitter tags – what people are bookmarking about twitter lately

TopTweet – This site provides you updates from the top Twitter personalities.

Twitter100 – Allocates a box to each of the people you follow on a single page and displays their latest tweets.

Quotably – Shows Twitter user’s conversations in a threaded conversational style making it easier to follow.

Tweetburner – Tweetburner lets you track what happens with the links you share on Twitter.

TwitterLocal – This shows you all the tweets on a google map from your local area and generates an RSS Feed of tweets.

TweetCloud – Let’s you make a tagcloud from a twitter user, also indexes del.icio.us

Tweetpeek – You can create a group Twitter feed with Tweetpeek.

Twitku – Mashup of Twitter, Jaiku, and Pownce public timelines. You can also post from its interface.

TwitterNotes – You can create notes on TwitterNotes using Twitter and tagging your notes.