Expat Web Directory provides listings of expatriate discussion groups and forums for most countries, including South Korea.
Expat.com is a worldwide directory of blogs related to expatriate life, with listings for South Korea.
Expatriates.com is an online community for expatriates, has a section on South Korea, with classified ads, links, and a bulletin board.
Korea4Expats is a solid resource for answers about legal and employment related issues, and much more.
Tour2Korea is the official site of the Korea Tourism Organization.
Lonely Planet Worldguide provides a lot of useful travel information, with a section on Asia, including South Korea.