Google checkout and Paypal save you the money of having to have all the gateways, SSL Certificates and is a perfect way to checkout for a small to mid sized business or start up. Nothing extra is required for hosting either. You can use the paypal shopping cart too. If you need more sophistication you could use Mal's

Other carts are:
http://www.zencart.com/
http://www.cubecart.com/site/home/
http://www.oscommerce.com/