algolia.com — Hosted search solution with typo-tolerance, relevance, and UI libraries to easily create search experiences. The free "Build" plan includes 1M documents and 10K searches/month. Also offers developer documentation search for free.
CommandBar - Unified Search Bar as-a-service, web-based UI widget/plugin that allows your users to search contents, navigations, features, etc. within your product, which helps discoverability. Free for up to 1,000 Monthly Active Users, unlimited commands.