FeatureKong is a hack of Bugzilla 2.13 to track features, requirements, and work estimates. We used it with much success, but it could still use some work. Hopefully, it will help you.
See the screenshots: query, feature list, and show feature. The two biggest changes from Bugzilla are the multi-line description (which is not cumulative like the comments which still exist) and the buckets (for tracking work estimates over two dimentions). Notice the links at the bottom of the feature list let you download as a spreadsheet (CSV) and include the buckets.
FeatureKong is distributed under the Mozilla Public License (MozPL)
Please send feedback to <email@example.com>
Both the current version and entire CVS repository are available.