I used Google Chrome's "extension" functionality. It is yet mature and development seems to be going on vigorously. It looks like an alpha version. Documents are also not sufficient yet. I'd like to pay respect to a few pioneers who don't hesitate to explore this wild and unmanned field.
Official document. Still coarse.
Building a Twitter Extension for Google Chrome
There's sample code.
How to install
It seems that extension functionality is not available in stables version of Chrome. "Dev" version or latest Chromium snapshot is recommended to install. Chromium is a flavor of Chrome for developers. Personally I suggest you use Chromium because it does not affect Windows registry. You can just unzip and use it. My Chromium is version 18.104.22.168 (24088).
To install TweetMonkey for Google Chrome:
1. First, you need to install Chromium.
2. Download TweetMonkey for Chrome.
3. Unzip it into C:\twmk directory. （This is just an example. In fact, any directory is OK)
4. Edit C:\twmk\tweetmonkey.html.
Put your Twitter information on the lines of username/password at the beginning of the source code.
var username = '(your twitter account)';
var password = '(your password)';
5. Run Chromium with --load-extension="C:\twmk" option. It can be convenient if you register it to a shortcut.
6. It's done. You should see a Twitter input box at the bottom of the Chromium window. (It might take a few seconds to show up)
How to use
An input box is always displayed at the bottom of the window. You can tweet from it anytime. If you click on the chain icon, you can enter the short URL of the current site. You can click on Update button to post your message on Twitter.