Look up a transaction on a hosted block explorer and two things happen: the operator learns which addresses you care about, and you take their answer on trust. The Mempool Open Source Project is built to be run the other way round. The repository was created in July 2019, and the project describes itself as completely self-hosted without any trusted third parties.
The focus is the fee market rather than the block history. The interface shows the unconfirmed mempool as blocks waiting to be mined, with fee bands, so a person deciding what to pay can see the queue they are joining instead of accepting a wallet's single number. The Bitcoin Dev Kit Foundation, which counts the project among its members, describes it as the leading open-source Bitcoin blockchain explorer and mempool visualiser.
Self-hosting is treated as the default rather than a footnote. The project ships one-click installs for Umbrel, RaspiBlitz, RoninDojo, myNode, StartOS and nix-bitcoin, and the README recommends in bold that most people deploy their own instance. Everything is licensed under the GNU Affero General Public License, which is the licence that obliges a company running a modified version as a network service to publish its changes.
The economics are worth stating precisely, because "open source project" and "company" are both true here. The public instance at mempool.space is managed by Mempool Holdings S.A. de C.V. and powered by an outfit the site calls wiz and associates. No jurisdiction is stated on the site, so the field is omitted here. Revenue comes from a Mempool Enterprise tier selling higher API limits, co-branded instances and a service level agreement, and its named enterprise sponsors include Spiral, Foundry, Blockstream, Unchained, Bitkey, Metaplanet, Bull Bitcoin, Gemini and Exodus. The project is also a member of the Bitcoin Dev Kit Foundation. No individual maintainers are named anywhere on the site, which appears to be deliberate, so this entry lists none.