Repos for the ps2dev toolchain -- the stuff that used to be on ps2dev.org.
"This article is an introduction to home-brew PlayStation 2 Development also known as PS2DEV." Up to date as of 2008-ish (i.e. it talks about FreeMcBoot).
Marcus R. Brown's site on PS2 homebrew, now defunct, but with some useful information: how to add a serial port, how to patch the Linux abstraction layer so you can use the whole of a memory card, and the details of the "Independence Exploit" (using a buffer overflow in the PS1 emulator) that used to be used for homebrew. Reading between the lines here you can see how he came up with the exploit too...
The Git repo for the current Linux port to the PS2 -- it's a 2.6 kernel rather than the 2.2 ones in the original PS2 Linux kit. The patches actually don't look too horrible, although I doubt there's much impetus to get this upstream now.
"Collection of tools to remotely debug PS2 applications, including commercial PS2 games". Used by the current PS2 Linux port.
The various generations of PS2 homebrew development hacks.
Lots of PS2 homebrew tools; he's the current maintainer of FreeMcBoot, but there are various hardware drivers and a hard disk diagnostic here too.
This appears to be the centre of PS2 homebrew development these days -- and the tools have improved quite a bit over the last few years. In particular, there's FreeMcBoot which you can just drop onto a memory card, or FreeHDBoot which you can dd to a hard disk (probably an easier option if you have a network adapter and a spare ATA disk -- so it's what I did). Discussion of other tools including the Linux kit and updated Linux ports here too.
Build your own XT from scratch -- designs for a backplane, processor board, various peripherals (including an unusually fancy graphics card), and so on.
Some great reverse engineering, and a couple of interesting homebrew CPU designs: a 14500-ish 1-bit CPU using an EPROM, and a 24-bit extension to the 6502.