Currently my favorite way to get HD material on my computer is with a capture device. Most capture cards are used to capture ATSC signals but many of them also support QAM, which isn't as useful as it should be, because the cable companies often encrypt most of their channels. Capture cards are available for most operating systems including Mac and Linux. Capture cards for computers are nothing new, in fact they have been around for a few years and are available in different forms, from PCI cards to USB adapters. Some hardware manufactures supply software to support the hardware like ATI or EyeTV. You can also use 3rd party products like Windows Media Center edition or MythTV. The 3rd party products tend to have more functionality as well as better integrated features. Some cards come with MPEG2 decoders which takes the load off the CPU or GPU, which means you can watch HD on an older PC, but we'll get into playback a bit later.
Currently I am using a Avermedia A180 with Windows MCE to capture OTA material. I use this because it allows me to stream HDTV from my HTPC to my Xbox360. The other reason is because the Avermedia is one of the least expensive cards available at ~$76.00. If you need a card with a MPEG2 decoder you can use MyHD MDP-130 or if you don't have any open PCI slots or are using a notebook, you can use a USB capture device like the Cats eye V-Box which is available with or without a remote starting at $100.
If you're a Mac guy like me, then your options are not as plentiful or cheap, but they are still good. You can use the EyeTV500 from Elgato which sells for about $349. Linux people are not left out in the cold on this one either; among other cards they have the pcHDTV's HD-3000 which sells for about $169. Most of the software to support these cards have some sort of interactive program guide so it is more like a TiVo than a VCR. Most do not charge extra for the guide info like TiVo does either, but they also don't include some of the TiVo specific features.
These are all great options for most but some people have another option which doesn't require a capture card at all. If you have another HD device like a Cable STB with a 1394 (Firewire) port you can capture HD by plugging it into your computer. There is an FCC mandate that requires all cable companies to supply their customers with a STB with a functional 1394 port, but unfortunately it isn't always easy to get. Others might have a TV with an integrated ATSC tuner and a 1394 port, and lastly there are 1394 ports on HD camcorders. Unfortunately the device manufactures don't supply drivers for some of these devices, but some intelligent person wrote one for us.
I used to use this technique before I bought an HDTiVo and tried to help others figure it out by writing this HowTo over at the AVSForum. There are also ways to integrate this capture technique in with windows MCE or Linux so you have DVR functionality. If you're using a Mac you can follow this HowTo using the Firewire SDK from Apple which is also available at the AVSForum. Linux is another process entirely; one that I haven't spent the time to make it work. I do know that it is supported by MythTV. Before you run out and get everything going, keep in mind that most of content is encrypted with 5C which makes it impossible to record, even if it is marked copy once. This is what keeps me from replacing my HDTiVo with a Windows MCE or MythTV.
Downloading or streaming HD from the Internet is finally possible, but not wide spread. MovieLink, which is only available for Windows users, has a few HD movies. You can also download free samples online as well as a few IPTV shows. DivX has some cool samples and there are HD Movie Trailers from Apple. You can also check out mariposaHD which is a IPTV HD show. Downloading isn't the only way to get WMV9 content, you can also buy DVDs like the T2 extreme edition.
The last way to get HD content for your computer is a bit more advanced and the legality is questionable. My favorite of those is HDTiVo extraction, which requires you to modify the kernel of your HDTivo so that you can enable the usb ports and connect your HDTivo to your network. Once you have basic network services up you can use TyTools for the pc and TivoTool for Mac (sorry Linux users) to extract your shows. Unfortunately, the USB drivers that the HDTiVo come with are only USB 1.1 so the throughput is very low. The authority on modifing your TiVo is the dealdatabase forum. Some say they don't take kindly to newbies so you may want to start at the Yahoo's HDTiVo group. If you don't have DIRECTV or you are interested in other ways to record you should check out 169Time or Nextcom. As always you can check the AVSForum Recording forum for more options.
Now that you have the HD content you'll want to play it on your computer. As you can imagine this can take alot of proccessing power. My first HTPC was a Pentium 3 1.0Ghz CPU with a Radeon 9200 pro. Using a DirectShow filter filter that effieciently took advantage of DXVA (DirectX Video Acceleration) I was able to watch 1080i sourced material at full resolution and frame rate on my HTPC. Admittedly I was not able to do anything else on my computer at the time because the CPU was at 100% utilization. Mac users aren't so lucky, for whatever reason Apple doesn't make it easy (or possible) for developers to take advantage of the GPU for MPEG2 decoding and as a result my Mac Mini can only play 1080i material at about 25fps with the most efficient codec. Most of the latest video cards support hardware acceleration, but it is a good idea to check before you make your next purchase. If you don't want to use the GPU to help you decode the stream then you're going to need at least a 3Ghz proccesor or equivalent. You will also need to have the correct video and audio CODEC installed. Most computers today come with DVD software that includes an MPEG2 CODEC, which is what ATSC is encoded with. Downloaded content will need a special CODEC depending on what was used to compress it. These are available for free but do require installation. The main CODECs you will want are XviD, DivX, H.264 and WM9.
I'll warn you in advance: these take even more power to decode than MPEG2. On Windows all you have to do is install the software and you'll be able to watch video encoded with these formats in any software that supports DirectShow. You can also try VLC which can decode just about any format and runs on all platforms, not to mention the fact that sometimes it is more effecient than the native CODEC. Another option is the free AC3 CODEC AC3Filter. VLC also supports Mac and there are Quicktime plugins for the other CODECs. Finally, you can also use EyeTv's playback software even if you don't have the hardware.