Edit: I want this information for a reference page on floating-point-related problems and alternatives to use when binary floating-point is inappropriate: http://floating-point-gui.de/
There's been a "port" of the Java
BigDecimal class (I think it's here: http://freshmeat.net/projects/js_bigdecimal/ ) for a long time. I looked at it a long time ago and it seemed kind-of cumbersome and huge, but (if that's the one I'm thinking of) it's been used as part of some cryptography tools so there's a decent chance that it works OK.
Because cryptography is a likely area to generate a need for such things, that's a good way to snoop around for such packages.