There is an alternative, Perian. Perian is a free, open source QuickTime component  that adds native support for many popular video formats.

The latest version, 1.2.1,  supports QuickTime X in Snow Leopard  and the following formats:

• File formats: AVI, DIVX, FLV, MKV, GVI, VP6, and VFW
• Video types: MS-MPEG4 v1 & v2, DivX, 3ivx, H.264, Sorenson H.263, FLV/Sorenson Spark, FSV1, VP6, H263i, VP3, HuffYUV, FFVHuff, MPEG1 & MPEG2 Video, Fraps, Snow, NuppelVideo, Techsmith Screen Capture, DosBox Capture
• Audio types: Windows Media Audio v1 & v2, Flash ADPCM, Xiph Vorbis (in Matroska), and MPEG Layer I & II Audio, True Audio, DTS Coherent Acoustics, Nellymoser ASAO
• AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3
• Subtitle support for SSA/ASS and SRT

The error box you will see when can’t play video in QuickTime X:

Download Perian plugin

QuickTime X – all video formats support is a post from: My MacOS X

If you have made the move to Snow Leopard then you are using the new QuickTime X. There are a few keyboard shortcut that can help you while using the new application.

here’s list how you can navigate the video from the keyboard:

• Spacebar
  Play and pause video playback
• Option-Up Arrow
  Increase volume to maximum volume
• Option-Down Arrow
  Mute audio
• Up Arrow
  Increase volume level
• Down Arrow
  Decrease volume level
• Command-Left Arrow
  Rewind movie, you can press multiple times to rewind the movie faster
• Command-Right Arrow
  Fast forward within the movie, with audio, again you can tap multiple times to fast forward in the movie at a faster pace
• Left and Right Arrows (With no modifier keys)
  allows you to scrub the video in slow motion, viewing in either rewind or forward frame by frame
• Option-Left Arrow
  Go to the beginning of the selection of the movie.
  (Note: When using the Option-Arrow combination, the cursor will go from the end of the movie to the previous cursor position, to the beginning of the movie.)
• Option-Right Arrow
  Go to end of the selection of the movie
• Command – I
  Show Movie Inspector (Source, Format, FPS, Data Size, Movie Size in pixels )
• Full Screen Mode
  Command – F
• Windows Sizes
  Command – 1  – Actual Size
  Command – 3 – Fit to Screen
  Command – +   – Increase Size
  Command    -    – Decrease Size

QuickTime X Keyboard Shortcuts is a post from: My MacOS X

One of the preferences is greatly missed was to automatically start playing a movie when it was opened.

But you can re-enable that behavior with the following Terminal command:

defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1

The movie will now start when opened without you having to press the Play button.

Enable autoplay of movies in QuickTime Player in Snow Leopard is a post from: My MacOS X