- http://drupal.org/project/double_field - two textfields (in one) with different widgets - eg text for amount and dropdown for unit.
- http://drupal.org/sandbox/entrigan/1194886 - decicated quantity/unit field (patch with extends it: http://drupal.org/node/1457990#comment-5656018)
- from commerce module: http://drupal.org/project/physical - especially for weight and dimensions