How can I change the 'active' class in main menu to something else? 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';

make it 

$class[] = 'current';

check API: 

http://api.drupal.org/api/drupal/includes!theme.inc/function/theme_links/7

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