Unlike ZIP or RAR, CSO allows emulators (e.g., PPSSPP) and custom firmware (CFW) PSPs to read compressed data on-the-fly without decompressing the entire file to memory.
It wasn't hosted on a flashy site; it was buried in the "Archive" section of a dying forum, hidden behind three layers of dead links and a password known only to those who still cared about the click of a UMD drive. The Last Memory Stick cso psp archive free