Bitcoin Core version 0.18.0 is now available from:

https://bitcoincore.org/bin/bitcoin-core-0.18.0/

For the release notes please see the git repository:

https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-0.18.0.md

Preferably use the above download link, not the links provided by GitHub to download the source tarball, as the release tarballs are generated deterministically whereas GitHub's are not.