Couple more options (programs):
Boxee - Primarily online content...I just started using it. It's nice because it pulls in video from a bunch of different internet sources (including Hulu and most major sites for TV networks). You can search for a show you want to watch and if it finds it, it will display all sources available. As an example I used it to watch an episode of Burn Notice off of USA's site the other night...and it was also available off of Hulu.
XBMC - This is more about your own local content, though they do have some plugins for online sources. The online portion isn't quite as smooth of an experience as Boxee is though. I prefer it when viewing my own content...much better interface for that.
I can definitely recommend a Roku unit...it's not just about Netflix anymore. It offers a bunch of free net programming (through their channel store) as well as video rentals via Amazon Video on Demand. Get the $99 version and you'll have a bunch of
HD (no more than 720p at this point, that's all the Roku can produce) content even if you don't go for Netflix. I do also recommend Netflix as well. I subscribe and use only the online streaming via my Roku. Their library of videos is usually slightly older (a year or two) to really old shows. A much smaller library than the DVD's/Blurays they have available through the mail, but it's still quite a good collection.
I have Boxee and XBMC running on my HTPC, my Roku, an Xbox 360 (and the Zune TV and Movie Library, and my PS3 (with the PSN library). My NAS (Network Attached Storage) houses my movie and TV collection. I basically converted all my DVDs to x264 format so they could be played through XBMC (no discs involved). I get 4 out of the 6 major local channels (thanks to this forum) direct into my
HDTV. When I moved into my apartment my plan was to do Internet and
OTA only...with no cable (other than internet) or satellite to save money. I have to say right now...I really don't miss it (especially since I would be paying basically double on my cable bill).
I gave Media Center a try but I couldn't get it to recognize the Dell Tuner I had (one my dad gave me...it supports
HD Content). I found a program called Media Portal that supported it though and it also turns your HTPC into a
DVR which is handy. I haven't given it a try since I moved to my apartment though. Now that my TV isn't going through a
DVR it might be more useful to me since I won't have the control delay I had with the dual
DVR setup (Tivo DirecTV receiver + Media Portal).