Internet Explorer 8, Internet Explorer 7, Firefox, Google Chrome... So many browsers out there to choose from now and all have their pros and cons. Could be a web designer's nightmare, though, when it comes to web design.
Each browser has their own 'issues' with dealing with certain HTML code, CSS styles, etc. Be sure to test your web site in several browsers to make sure all your users see your site the way you intended.
While you are at it, test your website in several different screen resolutions as well. Though there are still some 800x600 resolution monitors out there, most are viewing your site at a higher resolution, but you need to think about EVERYONE.