Compatibility View of internet explorer 6 or higher websites

You can enable the Develop menu for Safari Preferences > Advanced > check bottom option. Use it to change User Agent. Or you can just use FireFox instead of Safari for older sites that still are IE dependent.

If the Web site was merely created by a clueless developer then Kappy's answer is correct. There are some websites that insist on being displayed only on Explorer but don't really need Explorer. However, it is more common that the lazy developer utilized Windows only technology and the Web site cannot be viewed - or at least parts of it cannot be viewed - without Explorer. If this is the case you'll have no choice but to use a Windows solution. Though becoming increasingly rare this does remain a problem and it is one reason why I use Parallels Desktop with Windows 7 as a guest operating system. This lets me run Windows on my Mac while still using Mac software. Apple's BootCamp is another solution but it forces us to boot into either Windows or the Mac which I find inconvenient.