I usually have apt-get or yum install both smplayer and kaffeine from the repositories; then they'll get all the necessary dependencies automatically. They install mplayer and xine as their respective engines.
As WK said, you'll need to have additional repositories enabled to obtain the licensed or otherwise dicey parts like codecs or libdvdcss. My copy of Ubuntu 8.10 had the non-free repository already enabled (rather surprisingly if you ask me*). For Fedora you'll want to visit the new
RPM Fusion site or
ATrpms.
Programs like
smplayer that rely on the mplayer engine work across platforms, though if you browse the
Playback Forum you'll see people have been working on custom mplayer builds for Windows and OS X.
Most any modern Linux distribution includes a number of torrent clients; I use
deluge because it's pretty lightweight, though nothing beats
ctorrent in that department.
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* This might survive legal challenges in the US because the repository list doesn't actually contain the proprietary software. It's akin to the DeCSS cases a few years back, which were mostly heard in California, not Federal, courts. Posting the source code to DeCSS was held to be a violation of trade secret laws, but the trial court refused to ban linking to sites where the code was available. I don't believe this particular aspect of the decision has ever been reviewed, though. Because the defendant had posted the actual code on a US web server, the CA Supremes never reached the linking argument.
The other legal alternative would be for Ubuntu to buy the rights to the proprietary codecs and just eat the cost of the licenses. A full pack of codecs in the GStreamer format costs about $50 from Fluendo. You still can't watch DVDs with this package; it doesn't contain a CSS decoder. I realize that since Mark Shuttlesworth could afford $65 million to go into space, he could probably afford to buy the rights to all the software in nonfree as well. I think the problem is more getting the rightsholders, particularly those governing the DVD and BD schemes, to negotiate. (He can probably also afford the attorneys to defend the decision to distribute the nonfree respository lists.)
Who maintains the nonfree Ubuntu repositories anyway? I'm only acquainted with the Fedora world, where the repositories are maintained by volunteers and hosted outside the US. I've noticed that you can download proprietary items in Ubuntu restricted from servers whose names start with "us." A quick DNS search told me that us.archive.ubuntu.com is actually hosted in Poland on servers in the canonical.com domain.