<script type="text/javascript">
    ExecuteOrDelayUntilScriptLoaded(init,'sp.js');
    var currentUser;
    function init(){
        this.clientContext = new SP.ClientContext.get_current();
        this.oWeb = clientContext.get_web();
        currentUser = this.oWeb.get_currentUser();
        this.clientContext.load(currentUser);
        this.clientContext.executeQueryAsync(Function.createDelegate(this,this.onQuerySucceeded), Function.createDelegate(this,this.onQueryFailed));
    }

    function onQuerySucceeded() {
        //currentUser.get_email()
    }

    function onQueryFailed(sender, args) {
        alert('Request failed. nError: ' + args.get_message() + 'nStackTrace: ' + args.get_stackTrace());
    }
</script>