Baseball fans probably already know about MLB.tv - it's a streaming video service, run by MLB, which gives you access to streaming video of most Major League games, for an annual subscription (actually I notice they reduced their prices a lot this year, which is cool).
I used to use it a couple of seasons ago. It kinda worked on Linux in the browser with RealPlayer for a while, but then it stopped. I figured out a way to find the stream URL and play it manually with mplayer, which was a big pain. In the end I kinda gave up.
Yesterday I was lucky enough to come across mlbviewer, a simple console tool which automates the whole process in a really nice way. Basically, you install mlbviewer, run it once, and it'll create a config file in ~/.mlb/config . You edit this, put in your MLB.tv username and password, then run mlbviewer again. It gives you a simple, console list of all the available games, you select the one you want to watch and hit enter, and it opens it up with mplayer.
It's extremely simple, and it works brilliantly. It's actually a much nicer and more convenient interface than the browser-based one MLB.tv uses officially; reminds me of much, much older single-purpose apps before everything was done via the browser.
I packaged it up for Mandriva: it's in /contrib/release for Cooker and /contrib/backports for 2008 Spring and 2008. I haven't tested it on 2008 but it should work fine there. It definitely works fine on 2008 Spring and Cooker. All you have to do to use it is what I wrote above.
It's also available for other distros, of course, and there's a great thread on the LinuxQuestions forums for discussions and problems, where you can probably get help with installing and using it if you use a distro other than Mandriva.