HD Loader is a homebrew program for the PS2 which allow users to play games installed to a Hard Drive, formatted for use with a PS2.
The games can be copied to the hard drive from an original game CD or DVD from within HD Loader, or installed using a PC and software such as HDL Dump or WinHIIP. The PS2 HDD uses a standard IDE interface and the HDD can be connected to almost any PC.
Information and Downloads:
HD Loader Wikipedia - History of HDL, compatibility changes and info on cloned versions.
PS2 Hard Drive Compatibility Chart - A list of reported HDDs that are compatible with the PS2
Game Compatibility List - A list of reported PS2 Games for HD Loader, USB Extreme and ToxicOS
HD Loader Compatibility List - A list of reported PS2 Games for HD Loader
SKSApps - Many tools, tutorials, skins for HDL
PSX-SCENE - Patching apps, game management apps and the home of most upgrades for HDL
Console-Exploits - Home of Winhiip and the original PS2 HDD compatibility list