"Smart Trim implements a new field formatter for textfields (text, text_long, and text_with_summary, if you want to get technical) that improves upon the "Summary or Trimmed" formatter built into Drupal 7.
After installing and enabling Smart Trim, you should see a "Smart trimmed" option in the format dropdown for your text fields. With smart trim, you have control over:
The trim length
Whether the trim length is measured in characters or words
Appending an optional suffix at the trim point
Displaying an optional "More" link immediately after the trimmed text
Stripping out HTML tags from the field
The "More" link functionality may not make sense in many contexts, and may be redundant in situations where "Read More" is included in set of links included with the node. But it's there if you need it.
Initial release is strictly for Drupal 7. No backport to Drupal 6 is planned."