While the PlayStation 2 has a powerful softmod in the form of FreeMcBoot, you still can’t beat a modchip for getting complete control over the system.
The Modbo – a clone/reimplementation of the no-longer-produced Matrix Infinity modchip – achieves everything that FMCB does (primarily the ability to run homebrew apps), while also unlocking the DVD drive to play burned PlayStation 1 and 2 games, imported games from other regions, and even makes the DVD player region-free. It can also change the console’s video output standard (NTSC/PAL/etc.), and is compatible with all versions of PlayStation 2 (including SCPH-900XX).
Modbo 5.0 improves over 4.0 by adding the ability to load ELF executables from USB on startup. Otherwise, they are functionally the same.
NOTE: The Modbo is a challenging mod, requiring twenty (20) wires to be soldered from it to the PlayStation 2’s motherboard. It is recommended for beginners to practice on something else before attempting it.