There's also an option in IE8 to view websites in IE7 mode, though it's not completely reliable, from what I hear.
It has worked well for me so far, for my limited usage, although I'm only using it to test a basic stylesheet for IE7 and lower. I've started using the IE meta tag, but I found that the display can still be altered by setting the version number in the tag. Without the tag, the compatibility button appears in IE8 (except when viewing locally).
<meta http-equiv="X-UA-Compatible" content="IE=7" >