Usenet
Newznab
Newznab is a standardized API used by many usenet indexing sites. http://newznab.readthedocs.io/en/latest/
Several presets are available, but most require an API key to be accessible.
Headphones VIP
Website: https://headphones.codeshy.com/vip/
Private Newznab based indexer run by Headphones developers
OMGWTFNZB
Website: https://omgwtfnzbs.me/
Torrent
Gazelle
Gazelle is a webframwork geared torward private bittorrent trackers. Focusing mainly on music. https://github.com/WhatCD/Gazelle/wiki/JSON-API-Documentation
Presets: Oprheus.network, REDACTED.ch, Notwhat.cd
Torznab
Torznab is a wordplay on Torrent and Newznab. It uses the same structure and syntax as the Newznab API specification, but exposing torrent-specific attributes and .torrent files. Thus supports a recent rss feed AND backlog searching capabilities. The specification is not maintained and supported by the Newznab organization. (The same api specification is shared with nZEDb)
At this point it's unlikely your favorite tracker supports this. We'll update this post once we become aware of other trackers supporting it. Additionally you can use Jackett. It acts as a Torznab proxy adding Toznab support for more than 100 torrent trackers, but uses website scraping instead of APIs.
Important/Disclaimer: Many torrent trackers thrive on the community and may have rules in place that mandate site visits, karma, votes, comments and all. Please review your tracker rules and etiquette, keep your community alive. We're not responsible if your account is banned for disobeying rules or accruing HnRs/low-ratio.
Waffles
Website: https://waffles.ch