I use these sites from time to time (not too often, maybe two or three times a year), but they have been useful in a pinch. If nothing else, they can spark ideas and you can then make your own materials based on that idea.
BusyTeacher – You can download materials for free without an account. Well-organized, huge variety of materials.
iSLCollective – Requires an account, but you can log in with your Facebook or Google account to make things quick and easy. I’ve found some really handy worksheets here.
Lanternfish/Boggle’s World – Free, no account needed. Tons of material.
Waygook – Korea-specific forum with tons of lesson plans, materials, activities, and games (of varying quality). There are discussion boards for sharing ideas, asking questions about Korean culture, tips for living in Korea, etc. Take the extremely negative posters with a couple grains of salt. Unfortunately, Waygook.org now requires a monthly paid subscription in order to access and download files; however, the forum and discussion boards are free.
KorShare – Created as a response to Waygook.org’s paywall, this is a good free alternative (and the community is generally more positive about life in Korea).