I learned German so I could watch the porn.
And read some books.
German is easy to learn, the new rechtschreibung is a bit silly.
Learning German will help your english grammar some too, mayhaps.
OP already knows a Germanic language. It’s this obscure little North Sea Germanic dialect called “English”.
Or it his word order shall completely upmix.
How about Esperanto?
I’ve always wanted to learn it, and you have the language background to catch most all of it easily, I would think.
Plus, then you can talk in code with William Shatner.
I vote Japanese, it makes your choices for manga a lot wider, it also allows you to read the works of Basho and Miyamoto Musashi’s go rin no sho the way they were written. I am and i enjoy it immensly also its a very formulaic language in some ways which is helpful when learing.
I’d say Latin too, really helps with all the other languages you know (English, Spanish, French) and theres a lot of literature, poetry, etc.
Only downside is that you’re not likely to be able to order a coffee more easily anywhere in the world with it. 
I agree with Nichol_storm. I came in here ready to promote my language of choice, German, but then realized I don’t really have a single logical reason. It’s just that I’ve fallen in love with it. Just go with whichever language intrigues you right now.
I’d say Latin, Arabic, Japanese, or Italian. Possibly Mandarin as well.
Well, first of all, all three languages you know (English, Spanish, and French) have a lot in common. If you’re looking for a similar language, maybe go with Italian or Portugese or Dutch (Nederlands). Portugese has the advantage that, if you play the guitar, you can sing Bossa Nova in the original, which is probably the best seduction tool ever invented by humanity.
German, in some senses, has quite a different grammatical structure. While, in French, you may have been exposed to case, in German it has first-class status. It’s also good for teaching you about English. Verbs with a preposition have first-class status as words. These structures exist in English as well, but hardly any English grammars talk about them. German also has a third subjunctive that is used to talk about hearsay, while Spanish and English only have two. The generation of compound words also has first-class status in German.
Or you could try Japanese. No gender.
I’ll go ahead and put in a vote for Russian, since you mentioned it. It’s got a cool factor about it. You get to learn a new alphabet, which is actually pretty easy to pick up and once you do it’s pretty easy to pronounce. Much easier than English. When I was studying German I had difficulties with some of their vowel combinations. They weren’t native sounds to me as an English speaker. Russian doesn’t really have any sounds that an English speaker doesn’t already know how to make.
You also said you’re into literature so Russian could keep you busy forever. There are so many Russians moving to the states these days there’s a good chance you could find a native speaker to tutor you.
Just for pure fun, I always wanted to pick up a Native American language, or maybe Gaelic. Not sure if you get as much literature out of those, though.
Well, if you’re doing this purely for educational purposes, you could do what I’m doing and try to learn Basque. It’s totally unlike any other language out there, and predates Latin. However, it also is spoken by only 800,000 people at most, and I don’t know of many teachers of it.
Also, what about Latin? You already have a leg up on it, as that’s where Spanish and French came from, and English has relations to it. However, once again, it’s not really a language you can speak.
If you want something useful, go for German or Japanese.
Well if you’re going to stick with spoken languages, I’d go with German or Dutch. As German has ties to English you’ll be able to see the historical connection ‘tween the two. I find that this makes the time you put in to the learning very rewarding. Additionally, you’ll be able to chat with the natives at the 2006 World Cup. Speaking Dutch is, well, just good 'ol fashion fun.
However, IMHO, it’s nice to get away from the spoken (or read) languages and study sign languages. ASL would probably suit you best and it shouldn’t be too difficult to find instructors in/around any major city. Knowing a signed language is incredibly useful (as there really is no lingua franca for dealing with the deaf), you’ll have the chance to learn about an oft overlooked segment of society, and will be linguistically challenged/engaged as well.
Latin. This system is really cool - everything in the books are written entirely in latin, so you start reading latin from page 1, and you pick up grammar as you go along.
This is what I would suggest as well. Not only is it useful and interesting to learn, it’s a beautiful, visual language too. I took two semesters of it back in college and would love to be able to study it more.
And I have made my decision - I will learn Russian! I went our out and bought a good primer and lexicon today and I will order the Pimsleur cds soon too. Of course after a while if I find it too difficult or unrewarding I could change my mind. Also goodbye everyone. I have only been here a month but since I don’t believe in paying for internet content I must go. It was a very cool message board - one of the best I am a part of and I wish I would have found it sooner - but there are too many out there to pay even a dollar for one - so goodbye.
Too late to cast my vote for Elvish, then. Lissenen ar’ maska’lalaith tenna’ lye omentuva.