I'm doing Spanish Latin America and Mandarin on Pimsleur and Rosetta.
I would recommend a dual approach because Pimsleur is instant conversational and Rosetta helps with vocab, reading, pronunciation etc.. plus mixing it up makes it more likely you won't give up.
The latest Rosetta version also has an audio addition that you can put on your ipod etc.
What other languages? Well depending where you live and what you do or plan on doing.
I chose Spanish since it it's the most widely used second language in the US and Mandarin because it seems pretty likely that those fuckers are taking over the world as well as the Net shortly and no matter how many years they learn English at school, most people prefer to communicate in their native language if possible.
India isn't far behind so Hindi would be a good third one if you have the time and DEDICATION.
