FreeLoader is a program for the GameCube that allows the playing of games from other regions. It's a popular tool in the United Kingdom, where PAL game releases are often far behind their American counterparts. Obviously, American games are perfect for British gamers because they don't require translation, but UK games are usually translated into a number of other European languages before release, which is time consuming.
FreeLoader is used as a boot disk - after loading, it asks you to insert your game disc, and the game runs - a short and painless exercise. It's rumoured that games are starting to appear - such as the Japanese version of F-Zero GX - that do not work with the current version of FreeLoader, but it's likely a new edition of FreeLoader will be released to address this.
The GameCube will require a separate memory card for every region used - a US game cannot save to a card that has been formatted for Japanese games, and so on. Most importers will require three cards - UK, US and Japanese. You can reformat a card from any region, so three UK cards will suffice.
The most common use of FreeLoader is to allow UK gamers to play Animal Crossing, which has not seen a Eurpoean release despite popular demand. The two are often sold as a set, and since Animal Crossing comes with a free dedicated memory card, there's no extra cost involved.
Servo5678 notes that US gamers are using FreeLoader to play UK copies of Doshin the Giant, Nintendo's rather odd God sim, which it appears failed to get a US release. Interestingly, rumour has it that Nintendo used the sales of Doshin to help decide whether Animal Crossing should get a UK release, despite that fact that the two games are quite different and Doshin was never going to be a success in the UK market. Another reason Nintendo have given for the no-show of Animal Crossing in the UK is the effort involved in preparing the new Pokémon games for release.