Quicktime is ok for mac users as it's standard on macs but strongly advise windoze users NEVER to install Quicktime from apple as it completely takes over playback of nearly ALL video formats with it's own truly awful player. When you then decide to uninstall it, only then does it tell you that it can't be uninstalled without problems. The player install is also a very large download. You can install quicktime alternative from:
 http://www.free-codecs.com
but that too is still quite large (6 meg) and can't be completely uninstalled without probs.
I strongly urge video producers to NOT use quicktime for this reason. Stick to Xvid instead which is a far better cross platform standard. The codec is just 1 meg to download compared to about 12 meg for each of the proprietry formats.

Real Player - is the absolute pits. The massive spyware player bombards you with adverts and reports back to real.com every file that you play with it. Can't believe any one actually uses it and doesn't know this. Video producers, please DON'T post stuff in this hateful format. Again there is a real player alternative from:
 http://www.free-codecs.com

To all you browsers who don't want to mess about installing loads of complex video software, go for the mpeg option above which should play on every operating system with nothing extra required. I would though recommend browsers to install the Xvid codec as described above (even though there wasn't an Xvid version attached to the above post). Having this codec installed allows one to play high quality Xvid & DivX format files, quite a few of which are posted on IMC.