These are basic development tools that are getting hard to find for Tiger, and they're also not just for compiling mplayer. If you want to do any compiling or developing, these are the tools you want.
First one's the big one. Xcode from Apple. You can get this either from your install DVD or by downloading it from Apple's website. It's better to download it as you can get the latest version. You have to sign up for a free developer's account and then do a little hunting around on the site, but if you click Resources at the top, then find a Mac OS X Downloads link, you'll eventually come to the Downloads section. Tiger users want to download Xcode 2.5 Developer Tools. Leopard users, your latest version is I believe 3.1.4. The whole install will end up taking a couple of gigabytes, but on the plus side you now start developing all those awesome apps for Tiger you always wanted to.
Another tool you'll need to retrieve source code is SVN, a version control system that supplanted CVS as the popular choice. The Tiger version can be found by going to svnbinaries: Home, then clicking on "Need to download older binaries?", clicking the OS X folder, then choosing Subversion 1.4.6 Universal.
The last tool I have for you is Git, a version control system that supplanted SVN as the popular choice. Honestly, world, I wish you'd make up your mind. Both Tiger and Leopard universal binaries can be found here.
One thing going for Git, it has the funniest install ReadMe I've seen. It says, well, I'll just quote it:
I vote the latter.
GIT - the stupid content tracker
"git" can mean anything, depending on your mood.
• random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
• stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
• "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
• "goddamn idiotic truckload of sh*t": when it breaks
UPDATE: Oh, look. Somebody made Homebrew for Tiger and Leopard PowerPC.