I'm sure there are more reasons than just this, but it may come in handy to those that run into a 500 server error after enabling new modules or content-types.

Open up your .htaccess file and comment out "Options +FollowSymLinks" by adding a # in front of this line. Save, upload, and refresh your page.

Additionally, forcing php5 instead of version 4 may help this. You can do so by adding

AddType x-mapp-php5 .php
AddHandler x-mapp-php5 .php

to your .htaccess file.

Update - 1/17/2010

There are, of course, lots of causes for a 500 error. It was a little lame of me to suggest that disabling symlinks would fix them all. A lot of you that stumble upon this post just need to enable (or force) php5 instead of php4, as your hosting provider is probably supporting both and enabling 4 over 5 by default. This is assuming that you're on a clean install of Drupal, no crazy htaccess rules, and error 500 seems to have come "out of nowhere". Additional problems with 500 usually come from htaccess but can also exist because of other improper server settings -- php.ini, htpasswd files that are wrecked, etc. I think that 99% of the time, with Drupal at least, you'll be able to fix them by process of elimination inside your htaccess.

Another possible fix is to comment out *all* rewrite rules temporarily and see if the home page or admin page loads (?q= or ?q=admin)

Update 4/6/2010

This page gets a lot of traffic so I've disabled auth required for comments and allowed all anonymous users to post. Please comment with your own solution if you were able to find one and I will incorporate into this post.