I am a web developer and would like to debug a webapp/website on a Windows Phone device. It would be neat to have some kind of window where I can see the javascript console.
Windows Phone 7.5+ (IE9) and 8 (IE10) doesn't have a debug console but it's possible to add it to your web app via JSConsole by Remy Sharp.
Setting up a remote debugging session is straight forward enough. Visit jsconsole.com, get a unique session ID, add the session ID and JS snippet to your web app and you're good to go. More detailed setup instructions and video can be found on the website.
And although the JSConsole instructions doesn't explicitly mention Windows Phone support, a number of websites have confirmed it works.
Other tools, which may also be of use, are mentioned on a previous Stackoverflow question.
UPDATE: Internet Explorer 11 (IE11) introduces a new emulation tool which allows local (assuming you're running Windows 8.1) testing and debugging of websites for Windows Phone.