There are a lot of web browsers for different operating systems and cross browser testing on some is discussed
Windows– It can only run one version of IE hence can test only in installed version like IE 6, IE 7 or IE 8 but IETester program can display a web page for different versions like in IE 5.5, IE 6, IE 7 and IE 8. Install other browsers like Firefox, Safari, Opera or Chrome.
Mac – Safari web browser is provided but for other web browser use a virtualization software like VMWare.
Many cross-browser testing services websites are on internet for doing this task.