Obviously Linux comes with support for encoding and playing .ogg files out of the box, but on OS X you have to download a couple of tools first. To play .ogg files in iTunes, download Xiph.org's Quicktime Components (also, check out their FAQ for info on using it with 64-bit Macs). Afterward, you may notice your .ogg files are out of order when added to your playlist. To fix this, download Rescan QuickTime metadata 2.scpt.zip (the second one on the list)* from this page, unpack it, and add it to your home/Library/iTunes/Scripts folder (UPDATE: this script works in iTunes 7, but not 9, I didn't try it with 8). When you restart iTunes you'll notice the item "Rescan QuickTime metadata 2" in the scripts menu. Next, just highlight the songs you want fixed, choose the script from the menu, and your songs will magically be re-arranged in the correct order.
For encoding .ogg files, I'd been using Max for awhile, but recently it failed to look up a CD's metadata. Maybe the MusicBrainz server was down, but since Max was getting long in the tooth (last update 3+ years ago), I went looking for something newer. I went straight to X Lossless Decoder and saw their latest update (10/27/12) still supports Tiger PowerPC. Cool!
One thing about encoding in the program, by default it dumps all the files into the parent music folder you choose in the preferences. However, if you want to organize your files in separate /Artist/Album folders, you need to go to "Format of filename" in the General preferences and add "%A/%T/%n - %t" to the text field. The forward slashes will create folders.
And for playing .ogg files on an iPod? Rockbox will let you do that, though it's not yet compatible with all models. And if you have something newer like an iPhone or iPad, ditch it and get an Android device that isn't crippled out of the box ;)
*Also, this page has an iTunes script that combines the metadata rescan with ReplayGain, though ReplayGain requires vorbis-tools be installed "(e.g. through Fink)."