Microsoft has made available four sample master page sets compatible with the Application Templates for Microsoft Windows SharePoint Services 3.0 to showcase some of the customization options master pages provide. These can be downloaded here:

http://www.microsoft.com/downloads/details.aspx?familyid=7c05ca44-869a-463b-84d7-57b053711a96&displaylang=en&tm

If you want to have look at them pop over to the links below:

Block http://www.wssdemo.com/pages/master%20block.aspx

Clarity http://www.wssdemo.com/pages/master%20clarity.aspx

Horizon http://www.wssdemo.com/pages/master%20horizon.aspx

Reverse http://www.wssdemo.com/pages/master%20reverse.aspx

Have fun.