With Drupal's Feeds module, it's easy to import or aggregate any Drupal entities such as nodes, users, and taxonomy from different sources such as XML or CSV. Feeds does a great job at handling almost every situation, but sometimes the source data needs to be modified before it is saved. This is where the Feeds Tamper module excels. It provides an interface for you to make some of these common modifications.
One the most common modifications is splitting a comma-separated list of words into a multi-valued text field. You can also do string replacements like replacing every instance of the word "bot" with "Lullabot". Another great feature is the trim functionality, which is great for removing all those pesky trailing white spaces.
Here's a full list of options that are available with the Feeds Tamper module.
Here is an example of a string replacement, replacing every instance of "bot" with "Lullabot".
Feeds Tamper is currently in beta 2, is available for Drupal 6 & 7, and is in use on over 400 sites. Its maintainer, Chris Leppanen (twistor) has other feeds extension modules on drupal.org - be sure to check them out!