I'm sure this has already made its rounds, but I'm putting it here mainly so I'll have a record of it when I can't remember it again.

If your sites do not render correctly in Internet Explorer 8, you can use meta tags to tell Internet Explorer 8 to always render a site or page like Internet Explorer 7.

There are two options for doing this:

1. On a per-site basis
   You can add the HTTP header:
   <m eta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"/>
   
2. On a per-page basis
   You can add a special HTML tag to each document, right after the tag:
   <m eta http-equiv="X-UA-Compatible" content="IE=EmulateIE7"/>
   

For more details, see the Microsoft Documentation

*As Dan points out below, my blog filters won't let me display a "meta" tag, so I changed the name to "m eta" in the tags above. If you decide to copy and paste the above code, be sure to remove the extra space.