This site is a static rendering of the Trac instance that was used by R7RS-WG1 for its work on R7RS-small (PDF), which was ratified in 2013. For more information, see Home. For a version of this page that may be more recent, see HashQuote in WG2's repo for R7RS-large.
#"..." is rarely used. Of my 39 test Schemes, only Racket, Gambit, Larceny, IronScheme, Shoe, and and XLisp do something other than error out as soon as they see #" -- but in most of these cases, I don't know what they are actually doing.