If you use HootSuite and Firefox, there's no need to fire up HootSuite every time you tweet. HootBar adds two buttons to your browser: a little Twitter bird in the address bar, and a Hootlet button between the address bar and the search bar. To update your Twitter account, you just type your tweet into the address bar and click the Twitter bird button; the Hootlet button gives you more options. You can use multiple accounts with HootBar (just add --accountname after each tweet), schedule messages, and track stats. You will need a HootSuite account.
Note: This link takes you to the add-on's page at Mozilla.org, where you can automatically install the file into your Firefox browser.