"Help and friendly support for chronically messy people living in squalor. Conquer disorganization with humor and compassion." They use "squalor" as a synonym for unusual levels of mess. It's a treasure trove of accounts of people who went deep into squalor and came back out, plus a support forum for people who have hoarding tendencies.
Children of Hoarders
Mainly for children and other family members of compulsive hoarders. Has tips for dealing with hoarders, stories from people living in hoarders' houses, a locked forum that's supposed to be excellent, and help for hoarders themselves.
Messofthehouse's Youtube channel
Messofthehouse's father and little sister live in a duplex with MotH's stepmother, a severe hoarder who is in deep, deep denial. Twice a year, MotH goes home for the holidays and tackles one or two rooms to make a clean space for his father and sister, being careful not to step on his stepmother's toes. The resulting videos are amazing. If you're dealing with a hoarder, they're an example of how it's done; and if you're a hoarder, they're a kick in the ass to get your issues under control.
An A&E TV show that follows hoarders and their families through two-day interventions and cleanups. It focuses on severe cases, and many of the hoarders aren't ready to clean up and are being forced to do it by the law, so it can be depressing as hell.
Practical daily advice on decluttering your home. Many people swear by her. I can't stand her incessant grating tweeness, but a lot of the advice I find helpful turns out to have come from her via other bloggers.
Compulsive Hoarding Syndrome
The OCD Foundation's overview of hoarding.
Resources for Clutterers