Apache Bloodhound is a collection of plugins bundled with Trac. It includes some of the individual plugins suggested in earlier answers, like the AccountManagerPlugin.
The major plugins developed as part of Bloodhound are a very robust Multi Product implementation, full text search (based on Whoosh) with better navigation.
Ticket relations have also just been added.
Bloodhound keeps integrating newly released trac versions quickly, and all plugins interoperate as expected because they're purposefully bundled. It's also still compatible with most trac-hacks.
