I can’t vouch for the quality of these, but better than nothing.
NZB-SA - Haven’t signed up myself, but commenters seem to like it.
Usenet-Crawler - NewzNab Index with open registrations. 1,000 API hits a day.
NZBZombie – New NewzNab site but open registration.
NZBFinder – $10 gets VIP with 5,000 API hits
nzbTV.net – Basic NewzNab site
Nzbs2Go – Open registration
NZB7 – Open registration
MuttNZB – Open registration
FileSharingTalk NZB Kind of an old but not very well known NZB forum. Server is a little spotty, but still good.
NZBgeek – Forums based NZB indexer.
newzNZB - New, similar to NZBx.co. Broken, but still taking VIP donations. NZBsRus They apparently still up. Only available for $9.99 for VIP in BitCoins. Might want to try the ones above first. Now VIP only. $12 in BitCoins. NZBRoyalty - Not too bad for an NZB forum. They like to password a lot of their uploads, and they’ll disable your account if you never contribute. Registrations closed by admin.
Remember, account hoarding is key for the long term.
And if you know of any others, please let me know.