We have a SharePoint 2010 list of volunteers and I've created a workflow in SharePoint Designer 2010 that sends out email messages when a new volunteer is added. My client wants to have another workflow that does the same thing but only if one field has been changed. I see several options in the Conditions drop-down but none of those fit the bill. Is there another way to EASILY create a custom Workflow condition that bascially says 'if current item field value' has been modified?

Use an event handler on the list. You can find a starting example here

Use an event handler on the list. You can find a starting example here