Currently on my Mythbuntu box now. (Actually, I'm running it as a virtual machine on my windows box just to experiment/learn with before I make it a permanant install on another machine. I've tried the dual boot thing before and it's too much of a hastle. I loooooooove virtualization!)
I did a quick websearch and came up with
these recommendations. Haven't tried them all out myself. I do highly recommend VLC. And if you aren't already using Firefox, get it! Of course, I'm pretty sure Firefox is the default browser on Ubuntu now, so that shouldn't really be an issue.
Let me know if you try any of those listed apps and what you think.
One helpful app not listed on that article is Wine. Open a terminal and type "sudo apt-get install wine" (without quotes), enter the root password and go. When it's all done you should see Wine appear in the Applications menu. No, it has nothing to do with fermented grape juice. It's a windows emulator (even though
Wine
is
not an
emulator...yes, that's the stupid acronym) so you can run some applications that don't have a linux derivative that you absolutely need Windows for. Beware though, not all Windows applications will run in Wine, so your mileage will vary.
I needed to use Wine a while ago on a Xubuntu machine I was testing out. My kids wanted to play an online game that required Shockwave, and as there was no Shockwave player for linux (don't know if that's still the case) I just used Wine to install and run the Windows version of Firefox and installed Shockwave player in that. It worked great.