Also, not a programming language, but still a REPL: http://try.mongodb.org/

Okay, if HN thinks it should be there ... I'll put it there. It's on the list now.

Here's one that does Perl5, Perl6 (Rakudo), bwBasic, Emacs Lisp, Common Lisp, Scheme, Haskell, Io, Javascript (spidermonkey), Lua, OCaml, PHP, Python, Ruby, Postscript and Tcl !!!

* http://colabv6.dan.co.jp/lleval.html

PS. It was the sandbox used by Sunaba, a Perl5 Plack/PSGI online REPL: http://sunaba.plackperl.org/

Could be handy to add in a LaTeX-REP: http://mathurl.com/

While it's not a REPL for a programming language, it's one of the tools I use the most whenever explaining mathematics online. Oh and yeah, it saves your LaTeX-print as well.

I also like LaTeX Lab: http://docs.latexlab.org/

Actionscript 3: http://eval.hurlant.com/demo/

Type code in the top box; anything you Util.print shows up in the bottom one.

while not strictly a REPL http://wonderfl.net/

There's another (outdated) Arc repl at http://jonathan.tang.name/files/arclite/)

Missing the very cool LLVM REPL-ish thing: http://llvm.org/demo/

That is ridiculously cool! I wish it showed the output - it's just a "RE" otherwise. :(

Very little on lisp, but still cool.

http://biobike.org CommonLisp + knowledge representation + symbolic bioinformatics + visual programming; all through a web REPL.

Considering that the REPL concept has origins in lisp, I was disappointed that there weren't more online lisp REPLs out there.

That said, the online Arc REPL is nice.

I just found this not ten seconds ago and didn't see it on the list: http://lotrepls.appspot.com/

Tried to add this to the list but pbworks was playing up: http://jsconsole.com

Codepad.org is well represented on that list.