Seeking online WKT editor?

Take a look at Wicket, it's awesome:

http://arthur-e.github.io/Wicket/sandbox-gmaps3.html

The demo sandbox might be enough for you, if not you could probably use it to develop a simple Javascript app.

You can do this with OpenLayers.

Please have a look at the Vector Format's sample. Just select 'Well Known Text' from the drop down, and draw your shape. You will get your shape in WKT format.

For the Editing/Modifying functionality, have a look at the Modify Feature Example. This shows how you can modify an existing geometry.

You could copy the code to from both these examples to your own server and Integrate and edit it if required.