Running both Internet Explorer 6 and 7
When the first beta of Internet Explorer 7 came out, the biggest complaint was that there was no easy way to have it run as a standalone browser. You're pretty much forced to upgrade your whole system to use it.
I took the plunge anyway, deciding I'd rather run IE7, but I ran into all sorts of situations where I really needed IE6 for debugging.
There are a lot of hacks and instructions for taking apart the IE7 beta so that you can run it standalone. I've decided that the extremely easier solution is to just install IE7, then download the standalone of IE6.
Now that the Release Candidate is out, you probably don't want IE6 to be your main browser anymore, right? So why not just have it around for the few times when you really need it?