I had to do this for a client so I thought I would share it.

Here's some code to create a character counter for your text/memo boxes.

Note: I used KeyUp because TextChanged only fires when you LEAVE the control.

Note the use of GetText() to get the content. I struggled (stupidly!) with this trying to use the Javascript s.text property.  The controls have their own javascript methods. GetText and SetText.

<dxe:ASPxMemo ID="txtInfo" runat="server" Width="100%" Height="76px">
                <ClientSideEvents KeyUp="function(s,e){
                var ele = s.GetText();
                lblInfo.SetText(ele.length + '/4000 characters left');}" />
                </dxe:ASPxMemo>
<dxe:ASPxLabel ID="ASPxLabel1"  ClientInstanceName="lblInfo"  runat="server" Text="(maximum 4000 chars)">
                </dxe:ASPxLabel>

Hope you find it useful. If you have another way, please post it!

Hey Andrew,

Thanks for posting this solution, I'm sure others will find it useful. Btw, there is also a codecentral sample which you may find useful:

http://www.devexpress.com/Support/Center/e/E393.aspx