Textmate for Mac OS X has a folding feature which works in many situations. Very extensible and hackable, and has a small and active developer community around it.

No affiliation beyond satisfied customer and occasional bundle contributor.

Perhaps not the same, but quite cool. I can imagine that when another edition comes out, he'll put the corrections in, and perhaps (if he's honest) he'll put in a special page of acknoweldgements to anonymous coward, byte me, etc!