I just found out about the Yahoo UI Reset CSS tool/file and I\'m using it on my website. On my machine I have Internet Explorer 7 and Firefox 3.01 and my webpage looks the same
The purpose of it is that HTML(edit: the browsers have predefined properties for the elements.ty Zack) has some predefined properties for its elements like the built-in padding/margin for the paragraph element. The point of using the reset cheat sheet is to start with default values that are equal for all elements : size, margin, padding etc. I use the reset file in my project but not in its original form. You should decide which elements in the sheet to use and you can always assign other default values for the tags there.
edit: You can test your web-page by installing most of the browsers available. Here is the latest statistics representing the usage of browsers. My advice is to use Mozilla as your 1st choice testing browser and every now and then test the page on other browsers: IE6, IE7, IE8, Opera, Chrome and Safari (at least). Here is a reference to a discussion about multiple IEs on one PC. Best of luck!