When deleting any object field, salesforce prevents if its used in Apex code, workflow etc. but does not warn if used in Email Templates. Is there any app available to list field usage throughout salesforce ?
I'm not aware of an app for this, but what I do is keep a Force.com IDE project handy with everything in production (actually I was hitting the 5,000 limit so I have two projects, one with reports and one with everything else). Do a refresh then use a search tool, grepWin in my case, to find everywhere fields are referenced including email templates and reports. Works great for identifying field usage before deleting as you've asked here. Also works great for finding misplaced email templates and you can check the project into source control for some history too.