I have been using Resque quite a bit recently. It is a really nice asynchronous job system based on Redis.
Resque checks the queues for jobs to process in a fixed order. (In alphabetic order, to be precise.) This turns out to be a problem is you want predictable handling time for jobs. For example, consider a system which has queues aaa and zzz. If you add 100 jobs to aaa and 1 job to zzz, the job on zzz will wait a long time before being processed.
This problem is easily solved by just checking the queues in random order. Over time, any particular queue will be checked early so a few deep queues will not starve the other queues in the system.
resque-fairly is a Resque plugin which provides that behavior. Just install the gem, add require 'resque-fairly' and Resque will handle queues with approximate fairness.