BitTorrent's bencoding format, used in .torrent files, effectively uses netstrings-- but without the trailing commas, so it uses "5:hello" to represent filenames and similar.

Not sure if it counts as notable, but SCGI uses it too: https://python.ca/scgi/protocol.txt

Php serialized uses

   s:size:value;

For strings, which is pretty similar. Size is in bytes.

zurl and mongrel2 are using it.