I've found this add on to firefox incredibly useful for both character study and communication, especially when I don't have time to open a dictionary and go hunting for radicals.

Basically, its an instant Chinese character dictionary. You just have to highlight the character and a definition will instantly appear.

This can be a huge asset for MSN conversations and email communication with Chinese friends.

https://addons.mozilla.org/en-US/firefox/addon/3349

Check it out and let me know what you think!