[ocrow]

Seems like a coherent, sensible proposal, as one might expect from djb. Any notable protocols use them?

[Scaevolus]

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.

[asalahli]

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

[toast0]

Php serialized uses

   s:size:value;

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

[Asmod4n]

zurl and mongrel2 are using it.