Need I introduce the concept? By this point, it's pretty common knowledge, but a little recap can't hurt.
A Nuzlocke is a self-imposed challenge intended to make repeat playthroughs of Pokemon games more difficult for veteran players, with a sequence of rules designed to make you think hard about your choices and also make you grow attached to your teams. Starting life as a webcomic simply called "Pokemon Ruby Hard Mode", it grew into a widely accepted challenge for many people who've played these games over and over already. The name "Nuzlocke" comes from a Nuzleaf in the original comic who was named after the Lost character Locke, who met an untimely end far too soon, ultimately becoming a guiding spirit as the comic went on.
The basic rules are simple:
1) You must catch only one Pokemon per route or area.
2) You must nickname all Pokemon to forge a bond with them.
3) If a Pokemon faints, they are considered dead and therefore no longer usable.
Further rules can be applied depending on your preferences, ranging from forced set mode, no items in battle, and many other stipulations. A personal favourite of mine is the duplicates clause, allowing for "rerolls" if your first encounter is a Pokemon you've previously caught, for variety.
So, have you ever engaged in a nuzlocke before? Is it something you would consider doing in the future? Are you a madman like me who's done many of them over the years, even doing co-op "Soul Link" runs with friends?