Manx is a catalog of manuals for old computers.

Indexes content from bitsavers etc.

(Freemium) Search engine that indexes the full source code of web pages including HTML, JS, CSS, and webserver details.

The search engine calculates a score that aggressively favors text-heavy websites, and punishes those that have too many modern web design features.
This is in a sense the opposite of what most major search engines do, they favor modern websites over old-looking ones. Most links you find here will be nearly impossible to find on a regular search engine, as they aren't sufficiently search engine optimized.

Vendor blurb:

OpenSearch is a community-driven, open source search and analytics suite derived from Apache 2.0 licensed Elasticsearch 7.10.2 & Kibana 7.10.2. It consists of a search engine daemon, OpenSearch, and a visualization and user interface, OpenSearch Dashboards. OpenSearch enables people to easily ingest, secure, search, aggregate, view, and analyze data.