VLC player IS about the strongest player out there..(plays more files and formats), but "The KMPlayer" has an easier interface and plays 'almost-almost' as much and does more handy tricks while playing.

For I.E. and Firefox there is http://www.nirsoft.net/utils/video_cache_view.html


For Opera there is http://www.nirsoft.net/utils/opera_cache_view.html

For looking at your files, (IF you want to mess with the cache directly)and to help you moving & renaming them, I recommend http://www.freecommander.com/

Here's a screencapture of my Opera 9.64 cache in Freecommander, showing files sorted by size.....the one at the top at 1.88MB is probably some video file from several days ago. If I wanted, I could rename it, then click the red arrow in the center (blue is for 'copy') and move it to my 'songs' area in the bottom window.
(After doing it once or twice it seems easy)(at the top, I have several tabs for the locations of various browser caches 'locked' for easy access, as they are in arcane locations.)


there is also this for Firefox, and a search on "cache viewer" will get you most of the above links.