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