I would recommend using VLC - (click on link) which has all the codecs & support built into the player that you will need for any video or audio file you find on the Gnutella network. If VLC cannot play the file then an extremely high chance there is a problem with the file itself (corrupted or fake.)
QuickTime itself only comes with basic codec support. If you really wish to download plug-ins (codecs) for it then you should download codecs like Perian & perhaps Flip4Mac. See http://www.gnutellaforums.com/292880-post4.html
Via QuickTime Preferences you can also install other 3rd party software but not much there you will need for video files you find on Gnutella network.

Bitzi is a site where people can report files. Mostly video files are reported there. How to use Bitzi Web Lookup scroll down to post #6.

At bottom left corner of LW's screen you will see the connection bars. If grey or red then not connected.