A while ago, I put together a very simple module named String Overrides. It provided an interface that used some new functionality in Drupal 6 to easily replace strings without the use of the locale or i18n modules. For a demonstration of how it works, check out the video. It was designed for Drupal 6 because it relied on locale_custom_strings_en, which is new to Drupal 6. If you have a look at the bottom of your settings.php, you'll see a note about it. There were many requests for a Drupal 5 version of String Overrides, but it was never put into implementation due to its reliance on this core functionality.
Well, the brilliant quicksketch fixed this, creating a very slick backport to Drupal 5. This backport was so elegantly made that it still even uses locale_custom_strings_en, which was actually only meant for Drupal 6. For details on how he did it, see his post about it. So, if you have a site running Drupal 5, and you want a quick and easy way to replace strings, String Overrides is good to go. Thanks, Nate!