Free MCBoot, which I referred to in my last post on the previous page. Once installed on a memory card, it loads instead of the PS2's menu at boot, and allows you to do things like load ELF files from a USB drive (or, if you have a fat PS2 and everything to set one up, a hard drive), play MP3s and video files, and load some burned DVDs, as well as still allowing you to do your regular PS2 stuff. And you don't have to modify your system to install it, aside from possibly having to tape down some switches inside the console.
There are several methods of installing Free MCBoot, one of which involves swapping out a legit 007: Agent Under Fire disc with a slightly modified copy that includes a USB loader, allowing you to access the USB drive holding the Free MCBoot installer.