www.talktomeinkorean.com provides a lot of excellent (and free) lessons. New words and grammar points made easy.
www.harukorean.com is operated by TTMIK. Here subscribers can write their own sentences for each of the lessons and have them corrected by native speakers.
Dongsa is a verb conjugation website. You write the dictionary form of the verb in the search field and voilà, it’s conjugated and arranged according to tense and formality levels. You can also get the app for free in iTunes app store.
Lang-8: a site where you can write “journal entries” about whatever you please and a native speaker of the language you are learning will correct it for you. A part of the concept is that you also help others that are learning your native language by correcting their entries.
국립국어원: the National Institute of the Korean Language has their own twitter account called urimal365 where you can ask any questions you may have related to the Korean language. Using this service probably requires you to have a fair grasp of Korean to be able to ask the question as well as understand their explanation in Korean.