Its has been whipped up for a user who wasn't doing shipping or tax, so that's VERY IMPORTANT TO NOTE. This code is being used by one store but for all intents and purposes consider it UNTESTED, so make sure you test it thoroughly, as it might not work with your specific settings.

<script type="text/javascript">
// FOR TESTING PURPOSES ONLY
// FoxyCart v0.7.0+
// 2010.12.20
 
FC.checkout.InitCoupon = function() {
	var colspan = jQuery("#fc_cart_foot_total .fc_col1").attr("colspan");
	fc_cart_foot_discount_new = '<tr id="fc_cart_foot_discount_new"><td class="fc_col1" colspan="' + colspan + '"><a href="#" onclick="FC.checkout.AddCoupon(); this.blur(); return false;">Add a coupon</a></td><td class="fc_col2"><input type="text" name="coupon" id="fc_coupon" class="fc_text fc_text_short" value="" style="display:none;" /><a id="fc_coupon_apply" href="javascript:;" style="display:none;">Apply!</a></td></tr>';
 
	if (jQuery('#fc_cart_foot_discount_new').length == 0) {
		jQuery(fc_cart_foot_discount_new).insertBefore('#fc_cart_foot_shipping');
	}
 
	jQuery('#fc_coupon_apply').unbind('click').click(function(){
		var coupon = jQuery('#fc_coupon').val();
		if (coupon != '') {
			FC.checkout.ApplyCoupon(coupon);
		} else {
			alert('Please enter a coupon code.');
		}
	});
}
 
FC.checkout.AddCoupon = function() {
	jQuery("#fc_coupon, #fc_coupon_apply").toggle();
	if (jQuery("#fc_coupon").is(":visible")) {
		jQuery("#fc_coupon")[0].focus();
	}
}
 
FC.checkout.ApplyCoupon = function(coupon) {
	jQuery('#fc_coupon_apply').html('Loading...');
	jQuery.getJSON('https://' + window.location.hostname + '/cart?output=json&' + FC.checkout.config.session + '&coupon=' + coupon + '&callback=?',
		function(data) {
			fc_json = data;
			if (data.messages.errors.length > 0) {
				alert("We're sorry. An error occurred: " + data.messages.errors[0]);
			} else {
				FC.checkout.BuildCouponTR(data.coupons);
			}
			jQuery('#fc_coupon_apply').html('Apply!');
		}
	);
}
 
FC.checkout.BuildCouponTR = function(coupons) {
	var colspan = jQuery("#fc_cart_foot_total .fc_col1").attr("colspan");
	fc_cart_foot_discounts = '';
	FC.checkout.config.orderDiscount = 0;
	for (var coupon in coupons) {
		fc_cart_foot_discounts += '<tr class="fc_cart_foot_discount"><td class="fc_col1" colspan="' + colspan + '">' + coupons[coupon].name + ':</td><td class="fc_col2"><span class="fc_discount">' + FC.formatter.currency(coupons[coupon].discount, true) + '</span></td></tr>';
		FC.checkout.config.orderDiscount += parseFloat(coupons[coupon].discount);
	}
	jQuery(fc_cart_foot_discounts).insertAfter('#fc_cart_foot_subtotal');
	// Set the subtotal amounts
	jQuery('#discount, label[for=discount]').remove();
	if (FC.checkout.config.orderDiscount != 0) {
		discount_total = '<li class="fc_row fc_discount"><label for="discount" class="fc_pre">Discount</label><input type="text" name="discount" id="discount" class="fc_text fc_text_short fc_text_readonly" readonly="readonly" onfocus="this.blur()" value="' + FC.formatter.currency(FC.checkout.config.orderDiscount) + '" /></li>';
		jQuery(discount_total).insertAfter('li.fc_shipping_cost');
	}
	FC.checkout.updatePriceDisplay();
 
	// Comment the following line out if you want to remove the coupon line once a coupon has been added
	jQuery('#fc_cart_foot_discount_new').remove();
}
 
 
jQuery(document).ready(function(){
	var coupon_length = 0;
	for (var c in fc_json.coupons) {
		if (fc_json.coupons[c].hasOwnProperty('id')) coupon_length++;
	}
	if (coupon_length == 0) FC.checkout.InitCoupon();
 
	// If you'd like to display the "apply coupon" regardless of whether
	// or not a coupon has already been added, comment out the above lines and uncomment the following
	// FC.checkout.InitCoupon();
});
</script>

• http://forum.foxycart.com/comments.php?DiscussionID=3018&page=1#Item_3