Like RT, SqlLedger is a Perl web-based application that can be driven from the command-line and integrates with email.
We have done a simple integration and are interested in others.
When we are creating a quote for a prospect, we create an RT ticket with the prospect email as the requestor. When we have settled requirements, we have SqlLedger email a PDF of the quote to RequstTracker (as a -correspondence email). We use the RT ticket number as the quote number and modified SqlLedger to so the email's subject line has the ticket # in it. The RT mailgate adds the PDF to the ticket and forwards it to the prospect. Any replies the customer makes get put in RT. Very little work and very helpful.
-- MarkBucciarelli, Nov 15, 2006