I'd like to change the class in the main menu from 'active' to say 'current'. How can this be accomplished?
Assuming you are using Drupal 7.
You can overide theme_links in your own theme.
find the line:
$class = 'active';
$class = 'current';
by the way, I would suggest you not to change 'active', instead you can just add additional class for it. As many contributed modules use the default 'active' class. Such as SuperFish, Menu Block