MythTV for Linux users. GB-PVR for PC users who want free, and are tolerant of setup tweaking. BeyondTV or SageTV for people who can pay, and just want it to work.
Note that if you're using an external cable tuner the scheduler won't be able to tune the channel for you (unless you are setting up an external tuning link, which only works with a few set top boxes, or using something like IR Blaster).