It must be that time of year- lots of friends asking about HDTV again. Bottom line is that there are a lot of pitfalls and if you're in the market, you want to study up so you don't get taken. Here is a short list of online resources that I find useful:
ExtremeTech Guide to Buying HDTV
Recently updated, ExtremeTech does a good job of explaining the ins and outs and technical jargon of HDTV.
CEA's AntennaWeb.org
While the satellite and digital cable companies are really touting HDTV, in most areas in the US, you can get the overwhelming majority of HDTV programming for free (CSI, Alias, Monday Night Football, Jay Leno and PBS Prime Time for just a few examples). This snappy site from the CEA details what digital (and analog) channels you can receive and what kind of antenna you need. A nifty map even shows you where the television towers are broadcasting in relation to your home address.
CEA's Consumer Guide to HDTV
Ok, a lot of it is marketing drivel, but if you want to understand the difference between SDTV, DTV, HDTV, and other fun acronyms (and the logos that designate them), be sure to check this one out.
AVSForum
Every AV geek knows about AVS Forum. This is an essential bookmark. Before you buy ANY piece of home theater equipment, visit this site, and do a search. You'll find good discussion, reviews, and even exhaustive FAQ's created by avid users, technicians and the manufacturers themselves.
TitanTV
Good EPG data and lineup of HDTV programming in your area. Free signup required.
I'm always looking for add'l resources - these are the best I've found so far. Am I missing any other major sites? Let me know in the comments below.