Its easy to install a virtual machine on your laptop or desktop computer that will allow you to run other operating systems within your main os (ie: linux, windows running inside MacOS).
Virtual Machines:
Parallels
VMWare
The uncommon browsers may not be the largest part of your UserAgent slice in web stats, but don’t belittle the gain you will get in building accessible web sites and testing with them.
Common Browsers:
Internet Explorer 6, 7, and 8
Mozilla Firefox 2.x (recently released 3.0)
Opera
Safari 3.x
Shiira (new comer to Mac OS that strives for complete W3C compliance)
Unlikely Browsers:
Lynks (command line browser for the shell: linux, bsd, *nix)
Links2 ( some support for basic javascript and css, command line browser)
Gnopernicus ( open source screen reader available for Linux)
Its important that your clients have the ability to access your web site in a plain-text html fashion (no CSS or JavaScript) should their own personal needs require it.
You will learn a bit more about the importance of using semantic markup after looking at your site in Lynx for example. Using Gnopernicus is another great test to see how your web site holds up to text readers (otherwise known as “screen readers”). It is freely available on Linux.
There are a few handy tools for Firefox as well that will let you disable javascript, images, and css at the click of a button. You can also resize your window to see if it scales well down to minuscule screen sizes such as a mobile phone.