CoreAVC (using Media Player Classic or Windows Media Player):
CoreAVC is a H.264 codec developed by CoreCodec, and it is considered to be the most efficient decoder currently available. With this codec, you'll be able to playback high definition H.264 clips without the need to dedicated video acceleration. With it installed, you will be able to use any Directshow capable player to playback H.264 videos, including Media Player Classic and Windows Media Player.
The standard version of the codec will offer basic playback support, while the Pro version offers multi-processor acceleration, interlacing support and eventually, graphics card acceleration as well.
Pro: Very good compatibility and performance. Works in many media players. Has AAC decoder.
Con: Not free.
Instructions: Purchase CoreAVC. You will also need to install CoreAAC (AAC decoder), which is free/open source - you can find CoreAAC binary downloads here.
$59.99 (included as part of Nero 7 Premium package)
Nero ShowTime is a multimedia player that is bundled with the Nero Premium package, including the popular burning software Nero Burning ROM. The Nero package supports the encoding of Nero Digital format, which is infact their own H.264 codec, and Nero ShowTime can play back Nero Digital files and other H.264 encoded files.
The compatibility of Nero ShowTime is an issue because it is really designed to work with Nero encoded files, rather than files encoded using other H.264 encoders.
Pro: Easy to install. Good player interface.
Con: Not free. Poor compatibility with non Nero encoded H.264 files.
Instructions: Nero Download Links.