If you use Firefox, you can use Greasemonkey scripts to enhance your twitter.com page to add functions that Twitter still has not added to their site. Greasemonkey is a Firefox plugin that allows you to use Javascript to customize pages

Here are a few of the Greasemonkey scripts that I use to make Twitter a little easier to use.

Twitter Unread Marker

This is helpful when you have twitter open in a new tab or window. Instead of checking the website for new messages you can simply look at the tab to see how many new messages you have. Once you click on the window it will clear the unread message count.
http://userscripts.org/scripts/show/23229

Retweet This

Twitter has a reply and favorite button but no retweet button. This adds a retweet button below the reply button to easily retweet posts.
http://userscripts.org/scripts/show/26500

Nested Twitter Replies

Nice nested replies in Twitter.
http://userscripts.org/scripts/show/30598

Short URL Expander

Expand the shortened urls so you can see, before you click, where you will be going.
http://userscripts.org/scripts/show/37982

Display Popular Trends

Show in the sidebar the popular trends on Twitter at the moment.
http://userscripts.org/scripts/show/39353

Search in Sidebar

This adds the people and twitter search in the sidebar so you do not have to load another site before you perform your search.
http://userscripts.org/scripts/show/35610

I would like to find a twitpic previewer Greasemonkey script but haven’t found one that works yet.