SageTV seems to be the best PVR software at this time. That is to say, the most usable and well configured without requiring a lot of tweaking and in-depth customizing. Some people are happy with MythTV on Linux.

SageTV runs under Windows, Mac OS and Linux. I only have experience running the Windows version. While I don't like the fact the underlying OS can be quite unstable, I would recommend it above the others in general due to the greater support for a variety of capture devices.

The latest 6.1 and 6.2 releases are quite a bit better than all previous major versions. After completely wiping my system and installing a fresh/clean copy of XP Pro with SP2 it's running rock-solid. When I say clean I mean CLEAN. I have only installed the minimum drivers to get all the onboard hardware, graphics card and capture cards working. Plus a small ATI tweaker tool to turn on overscan and TweakUI to turn on auto-login and turn off Auto-Insert options for removable media.

In addition to installing SageTV you will want to install a third-party MPEG decoder. The one built into SageTV sucks ass even though no one seems to talk about in their forums. It positively does not handle an interlaced stream properly. Anyway, I'm just using ffdshow and it works beautifully. You can also install Intervideo Elecard or other commercial codecs for both regular and CSS-compliant decoding.

The product currently supports OTA ATSC, DVB (T, C and S), analog cards with hardware encoders, etc. A ton of cards. The Mac OS version doesn't support even a fraction and I'm not sure which products the Linux version supports right now (many less though).

Anyway, the only other options of relatively finished/polished products are MCE, BeyondTV and ReplayTV. Replay has a yearly charge of $20 which isn't bad. But all three of those products have an inferior UI in my opinion.