Your local cable provider more than likely offers some selection of HD channels. If you use an access box instead of just plugging in direct from the wall, the key is to make sure you already have the HD-ready version of the cable box. Of course, you'll also need an HDMI cable. The thing that none of these providers want to own up to is the fact that most (if not all) of their content isn't true 1080p HD. 1080i (interlaced) is about as good as it gets, and even that isn't very likely. Watching these shows on Blu-ray gives you better picture quality than their original airing. My favorite offender for this is Hallmark Movie Channel HD. Most of their stuff looks more like a VHS-quality transfer that drops every third frame of video.
The Best Networks
The major broadcast networks do a pretty good job all-around these days, aside from the non-1080p picture. The really shaky ground is under the cable nets. Thankfully, some of them make you glad you have your upgraded monitor. HBO, Animal Planet, and TLC program the most consistent high-quality video programs and have made an effort to get older non-HD programs up to snuff.
Building a TV Blu Library
Planet Earth is the easiest recommendation to make to anyone who's recently plunked down for an HD system. It's hands-down the hardest set to hate that's currently out there. HBO series (Rome, Band of Brothers, and Generation Kill among others) are another safe bet, as are CBS/Paramount's marvelous Star Trek: The Original Series sets. The quality you'll see is far beyond what you saw broadcast originally or now. Provided BD-Live finds its way to a faster-loading future, the idea of downloadable featurettes and features could add a fair amount of incentive to buying these sets, but we'll have to see.
Fans of Series Rejoice
ABC at the least has enabled a Season View option on LOST and other shows that allows the user to auto-play every episode on a disc and pick up at the exact moment they left off (the player remembers for you). I can't tell you how many shows I picked up on DVD that I went a couple of weeks without watching and completely forgot which episode I left off on and ditched.
More data area also theoretically means more episodes on each disc, so there'll be less disc-swapping. The jump to HD also means that DVD sets of existing shows will see their prices drop through the floor as Blu-ray versions become available.
The HD Guide is an ongoing series focusing on the evolving world of HD in the home: getting started, understanding the lingo, and appreciating the best (and worst) discs that are out there.