CNET has an excellent guide showing which programs on broadcast and cable/satellite TV are broadcast in high definition, it even goes so far as to tell you which are in 720p format and which are in 1080i.
Channels listed include: ABC, CBS, Fox, NBC, PBS, UPN, WB, Bravo HD+, Comcast SportsNet, Discovery HD Theater, Encore HD, ESPN HD, HBO HDTV, HDNet, HDNet Movies, INHD/INHD2, The Movie Channel HD, MSG Network, NBA TV, Showtime HDTV, Starz HD, and TNT-HD.
TitanTV Listings (thanks KCT)
(via pocketmediathoughts)
Agree on the CNET listings, but a better set of listings might be found at TitanTV.com. I wrote an article on how to use their listings to create a "TV appointment" on a Microsoft SPOT Watch.
Aside from the reminder functionality, TitanTV also has all of the HDTV information you could want; I've used it for the past 2 years to get all of my HDTV information.
KCT
KCT's Digital World
Thanks KCT, link to TitanTV added.
For sports, check out www.hdsportsguide.com they have a 14 day guide and individual pages for different sports. I agree on titantv.com, it's a great resource!