Okay, we all know that hacking core is bad... but if you need to make a really simple code change, sometimes it is much easier to hack core than doing it the *correct* way, that is, modifying the Drupal core program files vs. creating override functions located in template.php. Regardless of the methode you choose, it is important to have a centralized way to track the changes you have made to core, so you may verify that they are still correctly implemented, each time you upgrade the Drupal core code.
Here are some simple core hacks:
To remove the ":" (colon) after form labels:
/includes/form.inc ==> Remove : from !title: on line 2210 and 2213