Database Defragmentation

I'm looking for any information on existing products which will defragment a database (preferably without taking the database offline). There are of course many such products for disk defragmentation and I would like to find a similar solution for the DBMS - something other than doing a backup/restore. If anyone has any info regarding such a product or technique please respond to me directly - I will post a summary if the responses warrant. Thanks, Tom C e-mail: connolly@udel.edu