Two sheets - communication pictures

Developers who are at a distance from each other sometimes have to exchange views, which cannot be written with words, to sketch something quickly.

How do we solve this problem? Paint, Photoshop, Paint.NET, on-line services (underline the necessary), save, upload, link. It is necessary to correct - we repeat the process.

In light of this, I want to present a small but useful and convenient service - Two Sheets .
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What this service gives is two sheets, for yourself and for a friend. Everyone sees what the other draws. A new way of online communication.

Written without using Flash, only JavaScript (canvas tag) on ​​the client and PHP on the server. Data exchange - AJAX requests.

In the comments waiting for comments and suggestions for service.

UPDATE
Thank you all for the feedback! From the necessary for all I see (in importance):
1. Resize margins
2. Eraser
3. Adding text
4. Colors / line thickness
5. Graphic primitives
6. Loading pictures

Will improve in this sequence.

+ The idea of ​​a multi-user service. Made by