You can define your own buttons for HTML Editor's toolbar.
Let's make new button Insert Date for inserting current date to HTML Editor's content. We'll do it in a few steps.

Step 1

Toolbar's button should have width equal 23 pixels and height equal 21 pixels. Every button is associated with two images in gif format. The first image is associated with not pressed button and should have name ended with "_n". The second image is associated with pressed button and should have name ended with "_a".

There are two images for empty button in Editor/ed_styles folder of downloaded zip file.
They are ed_empty_n.gif and ed_empty_a.gif .

Using these images and PhotoShop we make two new images for our new button Insert Date.
They will be ed_date_n.gif and ed_date_a.gif .

These images should be placed to Editor/ed_styles folder.

Step 2

Now we should write JavaScript function that will be called on the button pressed. This function has one parameter: client-side HTML Editor object. The folowing code should be placed to your page.

<script type="text/JavaScript">
function insertDate(editor)
{
    editor.InsertHTML((new Date()).toLocaleDateString());
}
</script>

See For JavaScript Developers page for details.

Step 3

Now we add our new button to HTML Editor.

<ed:Editor id="editor" runat="server">
    <Buttons>
        <ed:HorizontalSeparator/>
        <ed:Custom OnClientClick="insertDate" ImageName="ed_date_n.gif" ToolTip="Insert current date" />
    </Buttons>
</ed:Editor>

See Custom toolbar definition page for details.

Here you see your new toolbar


See CustomButton.aspx example in downloaded zip file.
See also Custom buttons set.