mikebond: you have failed in majority of Slavic languages. That’s a challenge… 
I know… I studied most of them with Assimil courses only, which gave me some basic knowledge but not enough to speak or write with ease. I am trying Teach Yourself Polish now, let’s see if it works better!
Spanish
French
Mandarin
Russian
Arabic
There is just soo much in a language, I would try to learn at least one Amerindian language, maybe Quechua, and one African language like Swahili. To get a taste of those exotic language families.
I am spending all my time on German at the moment. I would like to get back to Spanish, which I was quite fluent in 3 or 4 years ago, Modern Greek (on LingQ please) and Mandarin Chinese.
Wow people want to learn so many languages 0_o.
At the moment, I’m only interested in learning Korean. Honestly , I’ve absolutely no interest in learning any other languages. I just don’t have the language bug in me =p. Perhaps if I ever live in a country other than korea or Canada I will want to learn an other language. For now Korean is more than enough. Other than Korean , Japanese might be interesting to understand. I do watch alot of japanese animation…