Note that Ren-C (a fork of Rebol that is only tangentially related to Ren) uses `blank` or literal `_`.
The language concept is a bit more fundamental than config files. Rebol (and Ren-C and cousin Red) is a fully homoiconic language that itself has tools for parsing and interpreting DSLs ('Dialects' to use the language's nomenclature) that use the same language rules. In essence you do indeed get the lexer for free.
There was a Parse-inspired project for JavaScript, but I'm not sure to what extent it was developed. There was also a Parse implementation in Topaz[1] (which itself is implemented in JavaScript) but is as of now unfinished.
It'd be difficult to implement as tightly in another language that doesn't have Rebol's (or a somewhat Lisp-like) free-form code-is-data/data-is-code[2] approach. Rebol and its Parse dialect share the same vocabulary and block structure that amongst other things: makes it easy to insert progress-dependent Rebol snippets within Parse; build Parse blocks dynamically (mid-Parse if needed!); build Rebol code from parsed data; develop complex grammar rules very similar to EBNF[3]. The article linked above[4] (and now linked again below :) does a good job of fleshing out these ideas and why it may remain a unique feature for some time.
Rebol is an elegant attempt at better using language to solve problems. It offers a rich vocabulary centered around words and includes natural representations of various types, such as URLs (that you can query), emails (that you can send messages to), times, dates and more. The main data structure (blocks) are instrumental in language flow and can be manipulated as data before being interpreted as code:
It is different and it's not immediately apparent what it brings to the table. However, that it does have this depth and is squeezed into a < 1 meg package without dependencies hints at why there are a few of us that consider it tremendously versatile and worthy of a more prominent place in the development firmament.
Literal notation utilised by JSON was indeed a part of Javascript, however JSON was a subset of Javascript specifically selected for cross-language messaging; by necessity pruning the parts of literal Javascript unsuited for this purpose and in part conforming to identical literals in other languages (see http://www.json.org/fatfree.html). It's unclear how without the limits set by the JSON spec that literal Javascript would have become a popular exchange format.
It took Crockford's Rebol-influenced eye to spot the potential for a minimalist though somewhat expressive exchange format based on elementary block structures, and near-genius to carve it out of the spartan confines of the Javascript syntactic swamps. Sadly not being Rebol, JSON does not include words (see 'Reserved Words' http://javascript.crockford.com/survey.html), dates, urls, email addresses (see http://www.rebol.com/rebolsteps.html); and will forever be burdened with comma-delimiters.