PSOLoad is the longest standing software DOL loader. It works by tricking a game called Phantasy Star Online (PSO) into downloading code from the network. PSO tries to connect to a central server if you want to play an online game, and the network protocol provides the facility to download an updated version of the game executable. By providing a fake server on the local network this mechanism can be used to upload arbitary code to the GameCube.
This technique requires a copy of PSO (Episode "1&2" only, not 1&2plus, not 3), a broadband adapter and a PC running the tool "PSOLoad" connected the the GameCube via Ethernet.
PSOload v2.0a for Windows.
PSOload for Windows also works in Linux using a recent Wine version without the need of additional DLLs. If you have a really old Wine version, like Wine 20030508, Azalyn explains here how to make it run.