Spanish would be useful in getting a job. (I’m from California)
Latin would be useful in reading historical texts, or writing fantasy novels.
And German would be useful for screaming like an enraged Nazi at telemarketers. 
And ditto that suggestion on Japanese for watching untranslated anime.
Ranchoth
ASL. I work with a deaf girl and I’ve learned basic signs from her, but I’d like to learn how to speak full sentences. The grammar of ASL can be tricky so it would be nice to just suddenly know how to do it properly.
Czech. So I can speak to my in-laws without an interpreter.
And I am learning it the hard way already, but I don’t expect to live more than 70 or 80 years so I doubt I have time. 
Japanese. I think I’m about halfway there already. But if I was REALLY fluent, I reckon I could make a killing on talk shows and such 
Finnish, Turkish, Hindi or Punjabi.
Latin.
Because…well, why the hell not!
SPANISH - I am a hospital intern, and it’s almost becoming essential for working with patients in metro areas. I, OTOH learned French, great language, but pretty useless to me now.
Well, I already speak preety good Korean, so I don’t need any help there. Ditto for Spanish. So I’ll say Putonghua (AKA Mandarin Chinese). I can get around in it, e.g. buy beer and ask where the post office is, but I’d like to be able to chat and have conversations.
The runner-up would be Arabic. That way I could read the Qu’ran in the original, get a much better paying job with the Feds, and be the nicer alternative when explaining Mideast culture to the TMs.
WV_Woman, assuming we’re in the same neck of the woods (I’m in western Pennsylvania), I’m afraid Japanese won’t make you that much money. Trust me. I speak it fluently, if a bit rustily, and I make my living programming computers. 
I’ve got Japanese and German well enough to have been a translator and interpreter, and I’ve studied a few other languages, so put me down for Italian simply because it sounds so beautiful, although I rather like the idea of learning Irish Gaelic.
CJ
Azari (dialect of Turkish) which is the mother tongue of most of my family (immediate and extended). I never understand what they are talking about and when they want us “kids” not to understand, they speak in Azari.
Japanese… not for Manga, but it would be nice to order parrts directly from Japan and not have to pay the ludicrous mark-up for JDM.
Either that, or Welsh, as it fits my personna for the SCA.
Another for Japanese. That way I’ll know if all those tourists are talking about me.
Plus it would help a LOT in getting a job here.
For those who are wondering, I found Japanese to be very easy to learn. I studied seriously (class 3 x a week plus at least one hour of study a day) while living there over five years. Well, the last two years, I slacked a lot.
Spoken Japanese is very easy to learn. The pronunciation is not hard and the words are phonetic, for the most part. The difficulty is the three (count’em) alphabets! Well, not even alphabets, entire writing systems!
Gambatte!
Tough call…either Arabic, or an Eastern European language. Probably Polish, since that’s where my roots are, but Russian could be fun as well.
C++
I know it’s not what you’re looking for, but that is my first choice.
If I had to pick a spoken language, it would have to be either Arabic or Russian.
Zev Steinhardt
Italian, without a doubt.
I speak enough English, German and French to get by. I’m planning on taking Spanish courses (mostly because it’s more widely spoken than Italian). But I’d love to be able to speak Italian.
Gaelic would be interesting, too.
So with full immersion and heavy studying, it took you five years? Sounds easy to me…
1st choice: French, for three reasons. One, I remember quite a bit of it from high school and college, and it wouldn’t take but a few years of study to develop at least conversational fluency. (When I was trying to learn some German for my trip back in April, I was amazed how often my brain, thinking “foreign language,” defaulted to translating English into French.) Two, it has some wonderfully earthy idioms. And three, many of the writers whose works I enjoy in English wrote originally in French, and I’d like to read them in their own language.
2nd Choice: Welsh. It’s an incredible sounding language, and there are few native speakers around anymore.
Ancient Sumerian. Or some other dead language that no one on Earth knows how to speak or read
Polish - to make my genealogy research easier.