What better way to remind us of the universe as a cold, uncaring vast expanse and the prevalent threat of sudden death than space sims. The tradition started with the '80s classic Elite has been carried on with many 21st century titles that focus varyingly on trading, mining, combat, or just flying around in the space sim version of lurking. Among these sims are many freeware titles, and now that I've recently done a run down of first-person-shooters still available on PowerPC, I figured I'd do the same for space simulators.
We should start with Oolite since it's the most directly inspired by the original Elite. Like Elite, Oolite is open-ended, meaning what you do in the universe is entirely up to you. Wanna be a pirate? Go for it. Out of the box, the graphics are somewhat rudimentary, but you can change this by installing among tons of expansion packs -- ships, skins, HUDS, sound packs, and more. The stable release requires Tiger and is a universal binary, but their development release seems to require Intel and Leopard, so it looks like PowerPC development for OS X is dead unless someone wants to take the source code and run with it. Debian users can install the development release from their testing repositories, but given the state of PowerPC graphics drivers on Linux...ugh.
Similar to Oolite but perhaps more elaborate is Vega Strike. Their OS X download is a version behind the latest release and seems stuck there. I think they're looking for packagers. One weakness compared to the others on this list is its startup time, which is disconcertingly leisurely. It's also more demanding in the graphics department, though you can tone it down in the settings to play fluidly on almost any configuration at the expense of prettiness. Like Oolite, this is open-ended, too, though the universe here seems a bit more populated. Unlike Oolite, it's so complex it's hard to get into and start playing right away. Thankfully it comes with an extensive PDF manual. The more you're into this genre, the more you'll enjoy this.
Finally there's Babylon 5: I've Found Her. If you think Babylon 5 refers to the TV show, you are correct! IFH is a fan-made game from a couple of Russian developers, probably the best fan-made game you'll come across. Unlike Oolite and Vega Strike, IFH is mission-oriented rather than open-ended. It puts you in the cockpit of a starfury and is mostly focused on shoot 'em up style action, though there is some exploring as well. Fan-contributed missions can be downloaded from the FTP link in this thread (the unpacked files have to be manually installed into the app package -- ooo, fun). There's also a Simulator mode for quick dogfighting action. For training purposes, of course. Not to satisfy any blood lust. That would be unseemly.
Another option is FreeSpace Open, which requires the purchase of the original FreeSpace 2, and you also need a Windows system to extract FreeSpace 2's data files. Instructions for installing this on OS X are at the FreeSpace Wiki, though from the look of Youtube clips, FreeSpace Open might be too graphics intensive for all but the fastest G5s.