The TicketRelations plugin enables you to define dependency relations between tickets.
TicketRelations is based on mastertickets by Noah Kantrowitz but removes the extra fluff (graphs) that came with mastertickets. The original version is available from https://github.com/coderanger/trac-mastertickets
The concept is the same, in that you define ticket relations using the custom ticket fields "blocking" and "blocked by".
To enable TicketRelations, add the following to trac.ini:
[components] ticketrelations.* = enabled [ticket-custom] blocking = text blocking.label = Blocking blockedby = text blockedby.label = Blocked By
TicketRelations is made for and tested on Trac 0.12. I have no idea if it works on older versions. The plugin is entirely safe to test and use as it will not interfere or alter existing data in any way.