How is it possible to extend GMail?

I'd like to write a gmail extension that would add a button on the email page (or a seclectbox, or anything). For information, the goal is to add a task into my task manager from an email content.

What would be the way of doing it ? A greasemonkey script is not an option as I would like to do it for Chrome too.

I don't know where to start.

T开发者_C百科hanks for any pointers, Mickael;

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Like the comment stated, Greasemonkey scripts have been integrated into Chrome. An alternative would be to develop an extension (for both FF and Chrome) similar to "Google Redesigned" - one of my favorite FF extensions.