The TIOBE Programming Community index is an indicator of the popularity of programming languages.

See http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html
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Popular for what? Hacking around, or "real" work? There are languages that are fun to fiddle with, but I wouldn't want to use them in a team environment. These are sometimes called WriteOnlyLanguage''''''s.

''Follow the link.  Read what it says.''

Quote: "Observe that the TIOBE index is not about the best programming language or the language in which most lines of code have been written."

It's mostly a '''"mentioned" tracker'''. Says very little about production, and even admits that. Thus, nothing reliable to distinguish between "fun" or "faddish" languages/trends and commercial production code. The best one can say about a language with a high score is that it's getting a lot of attention, and that could be primarily only "mental attention". Some of those languages are also common in academic environments, and thus college assignments etc. would be picked up in the "mention" tracker. 

APL, SmallTalk, and Lisp are examples of languages that people talk about a lot, but are ''not'' heavily represented in commercial and government production environments (outside of academia), although some claim otherwise as GreatLispWar illustrates.

''We have a shorthand phrase for languages that "people talk about a lot" -- we call them "highly influential".  What modern, popular programming language doesn't owe much to APL, Smalltalk, and LISP?"''

No doubt, but Tiobe is not very useful for distinguishing between "influential" and "used in production". We need a better tool for that. Some suggest using Dice.com for getting a better handle on production languages.
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Regardling PopularityOfLisp

Dice.com survey 6/4/2013:
 Lisp: 39
 Python: 4221
 Java: 17619 (May include some JavaScript)
 C-sharp: 8803
 Fortran: 47
 FoxPro: 49
 Cobol: 672
 Ruby: 2536
 Objective-C: 1742
 C++: 16698
 Sql: 25435
 Php: 3439
So Lisp has about as much market demand as FoxPro. What irony ;-) -t

''Heh. :-)  Where on dice.com did you find that survey?''

Basically typing in the key-word and seeing what the match-count is. FingerWare.

''Ah, then you should probably include Scheme, Racket and Clojure and add them to Lisp.'' 

I get about 200 for Scheme, but there's a lot of overloading with "database scheme" (schema typo?), "security scheme" (plan), etc. (If you want to keep a new language and its usage obscure, name it "The". There used to be a music group named "The" IIRC. That would be suicide in the Google era.)
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TypeHandlingGrid discusses the probability of having to use relatively-obscure languages in the workplace.
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CategoryEmployment