For video, I recommend VLC, but you'll also need the GStreamer plugins. For Flash, just go to any site that uses Flash in Firefox, and it will say "Plugins needed" or something, click on the puzzle piece icon that says plugin needed, and it will take you to a menu that downloads Flash and installs it. For the other codecs (wmv, mpg, etc), the easiest way to install them is probably Automatix, just add the Automatix repositories and download it. It makes installing certain things easy.
I'm pretty sure it's at
getautomatix.com but I could be wrong.
EDIT: It's .com not .org, I updated the link.