I'd recommend this player VLC - (click on link) _ (choose nearest mirror site) as it has in-built codecs & is very reliable & is Free. Another good one which is similar to VLC but can do the odd thing VLC can't is MPlayer (Click on link) & is Free. Good to keep both of these handy just in case.
And if you wish to change one of these to be your main video player, then see How to change default program to open files with in OSX


Also, you can expand QuickTime's range of files by installing Perian and Flip4Mac:

Perian: Perian - The swiss-army knife of QuickTime® components __ (Free / Donateware) ____ (An alternative plug-in codec for QuickTime to use instead of DivX, XviD or 3ivx. I highly recommend it. It has many abilities including adding subtitle support for QT.)

Flip4Mac wmv/wma plugin: Flip4Mac - Digital Media Tools for the Mac __ (Free / Shareware for version to convert to wma/wmv)


If you wish codecs for QuickTime player, then you could get DivX, _ XviD, _ 3ivx. And perhaps ffmpegX and FFusion. And for QT support for WMV files then Flip4Mac WMV Player ____ (* Warning, some of these codecs such as DivX, XviD, 3ivx are now shareware/trialware even just for playback. Perian which is free seems to be an easier option.)


Update QuickTime Plug-In Codecs & make sure the appropriate QT video & audio options are enabled -> Enabling & updating QuickTime Plug-ins


General gnutella network Video download advice: Movie download issues (click on blue link).

Note: re: Previewing; to prevent Preview files being created in your incomplete folder & taking up space which you need to delete later, instead I recommend going directly to your incomplete folder listed in LW Library window, find the file you wish to preview & select it & press Launch button. Usually need to have first downloaded 5-10% of a video to preview it (varies depending on video format & what % of start & end of video has been downloaded, since LW does not necessarily download files in sequential order. Sometimes not enough of the video information start & end & other info attributes has been downloaded for the player to recognise it as a video file. VLC is a much more reliable video previewer than QT is. However, do not repair an incomplete video if VLC asks to.



Perian enables QuickTime application support for additional Media Types:
AVI, FLV, DIVX, GVI, VP6, VFW, and MKV file formats
MS-MPEG4 v1 & v2, DivX, 3ivX, H.264, FLV1, FSV1, VP6, H263I, Sorenson H263, VP3, HuffYUV, FFVHuff, MPEG1 & MPEG2 Video, Fraps, Windows Media Audio v1 & v2, Flash ADPCM, Xiph Vorbis (in Matroska), MPEG Layer II Audio, DTS, True Audio
AVI support for: AAC, AC3 Audio, H.264, MPEG4, and VBR MP3
Subtitle support for SSA/A.S.S, SRT, SAMI