This was actually a nice thread in the old forum, so I'm reviving it.

My list (in the following order...):
-Mandarin
-MSA
-Some Persian
-Some Cantonese
-Well probably some of an Arabic dialect
-A lot of facts about Old Spanish
-Maybe French
-Well probably some Hindi
-Slightly maybe some Portuguese (most likely yes)
-Slightly maybe some Latin (most likely... idk, in the middle of yes and no)
-Slightly maybe some Nahuatl (most likely no)
-Slightly maybe some Russian (most likely no)
-Slightly maybe some Gujarati (most likely no)

I'm going to unquestionably learn well the first two (MSA mainly for writing though). I'd be happy with a lot of Persian/Canto but not perfect, an Arabic dialect is crucial to speak with people, Old Spanish and French are out of pure geekiness, I imagine myself speaking Hindi but it's not a journey I want to start for now. For the rest (Port./Latin/Nahuatl/Russian/Gujarati) it all depends on how things turn out (say, if I become a close friend of a Russian/Gujarati speaker or something I'd be more than delighted to learn it).

Mine:
German
Norwegian
Luxembourgish
Japanese
Cornish
Dutch
Those are the ones I'll most likely learn fluently, but I haven't fully committed to Luxembourgish yet.

Possibly, in order of possibility:
Manx
Latin
Quite possibly some Old Norse, it's pretty sexy
Maybe Persian, it's pretty cool
I also want to maybe learn either Latvian or Lithuanian, I can't decide which, and surely not both, and also a Finno-Ugric language, in this order: Võro, Estonian, Finnish/Hungarian, not sure which order they should go in, since there's an equal chance for both.
In reality, I doubt learning Võro is possible, because I don't think many (if any at all) resources are in English, and I have no idea about other resources for it in any other language.

Less possibly:
Mandarin
Arabic
Low possibility: Some Native American language, like Cherokee or Lakota.

Langauges I really want to learn:
Hindi
Tamil
Arabic-Egyptian & Lebanese

Possible Langauges:
Nepali
Malayam
Telugu
Urdu
Bengali
Tajik

Really interested in (but probably will never learn)
Khmer
Tibetian
Sign Langauge
Sanskrit

Well since my area of interest includes mostly history of East Central Europe I plan to learn languages of this region, even those I'm not particularly interested in. Just in case I change my signature these language would be:
Hungarian, German, Polish, Sorbian, Romanian, Slovenian, Lithuanian, Latvian, Albanian, Romany, and I'll try to improve my knowledge of Czech, Ruthenian, Macedonian, Bulgarian, Ukrainian, Belarusian and Russian.

I would also like to know languages that were used in these regions in the past: Latin, Koine Greek, Gothic, Old High German, Old Hungarian, Ottoman Turkish, Old Prussian, and improve my knowledge of Old Church Slavonic and Church Slavonic.

But I also would like to learn languages of other cultures I'm interested in: Old Irish, Old French, Old English, Old Norse, Classical Greek, Ancient Egyptian, Coptic, Hittite, Sanskrit.

There are couple of modern languages I would like to learn also, for instance French, Italian, Spanish, Finnish and Swedish, the former three because there is a large volume of literature available on the subject of palaeolinguistics and history, the latter two because I just like them.

I have my whole life ahead of me and I plan to live a very long one, and I'll spend most of it studying I guess. I can't tell how many of these languages I'll learn or even try to learn, but most of them require only passive understanding, and as I don't plan to really make anything of them except get to the knowledge I seek it is possible.

I'm currently learning Classical Greek and Old Church Slavonic.

Wow some of you guys have a lot of languages to learn. I'm not sure I'd ever learn a dead language. Anyway,

Learning:
Spanish
French

Started but not right now:
Hebrew

Might Learn to become a Romance language aficionado:
Italian
Portuguese

Interesting to Me:
Arabic
Icelandic

Currently learning:
Danish
German

Brushing up on:
Italian
French

Definitely learn:
Russian

Most likely will learn:
Míkmaq / Cree / Ojibwe
Hawaiian
Finnish
Welsh

Might learn:
Lakota
Icelandic
Luxembourgish
Mandarin

Probably not, but I really really want to:
Lebanese Arabic (no MSA for me!)
Modern Hebrew
Dutch
Cantonese
Xhosa/Zulu/Swahili
Biblical Hebrew / Ancient Greek / Aramaic

Unfortunately, that is out of our hands

@formiko: What a happy thought...

You're missing the best part!

I want learning/plan learn:
-Aleut
-Quechua
-Nganasan/Nenets
-Itelmen
-Japanese