Spanish is very useful and has a huge number of speakers. It is not too difficult to learn, especially if you already know French. The grammatical rules are similar, the conjugations easier although more numerous, and the pronunciation is certainly easier, as there are only pure vowel sounds and written spanish is almost entirely phonetic. Plus, you'll have no difficulty finding people to speak with, as nearly every urban and suburban area in the US has an abundance of spanish-speakers.
Outside the romance family, I've tried my hand at Arabic and classical Greek. The former could be useful, although it is quite difficult and you'd have a harder time finding people to practice with. There are relatively few good books for English-speakers to learn Arabic. Mandarin, I think, could be highly useful in the science and medical world (3/4 of the things I learn in my lab come through the grapevine of mandarin mixed with English at the lunch table in our conference room). I bet there are some really good books for English learners too. The downside is that it uses characters and not an alphabet, which could be very very challenging for us westerners to master with any proficiency in less than a few years.
Regardless of which language you choose, here is perhaps the best piece of advice I can give you. Get MSN messenger and a webcam. Find a website that links you with online 'penpals' who want to learn English, but who speak the language you want to learn. I practically taught myself to speak French like this, and I made some good friends along the way too. Skype is another great program you can use to speak directly with foreign buddies who can help you with your language acquisition, and vice versa.
Have fun.
