With every web browser on the planet rendering differently it's a constant battle ensuring that your beautifully and painstakingly designed web site looks just as gorgeous on all browsers. Personally I build a site entirely using Firefox and only at the end of the build process do I look at it in other browsers, curse and swear and then start to fix issues.

One of the next best things to having local installations is the Adobe BrowserLab (registration required). It allows you to view the pages of a given site in either two browsers side-by-side or "onion skinned" overlaying each other to see the exact differences. The tool does not, however, allow you to interact with the sites, so it's best used for display-only problems.