I don't recall exactly how I found this, but the longest single word domain (without hyphens) is llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch.com. If you don't believe me, click on the name and it will tell you as much.
For those of you who didn't click the link and are still with me, that's a 58-letter domain name meaning "The church of St. Mary in the hollow of white hazel trees near the rapid whirlpool by St. Tysilio's of the red cave," which is the long way to describe this Welsh town.
The scary thing is that this isn't even the longest town name in the world; that belongs to either a 92-letter town in New Zealand or a 163-character town in Thailand, depending on your source.
By the way, the townsfolk at Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch prefer to go by Llanfair, which is much, much easier to remember.