“NaCl (pronounced “salt”) is a new easy-to-use high-speed software library for network communication, encryption, decryption, signatures, etc. NaCl’s goal is to provide all of the core operations needed to build higher-level cryptographic tools.”
The main author is Daniel Bernstein, who has a long history of writing very secure and stable software.
The main page lists a paper that should be read that covers the design of the library.
Despite the proclamation, Sodium is Nacl, but a re-implementation sharing the same API. NaCl apparently only compiles on a few platforms, whereas Sodium builds on many platforms, including OpenBSD, Dragonfly BSD, NetBSD, FreeBSD, SmartOS, OSX, Linux, Windows, iOS and Android.
Not to denigrate the developer of Sodium at all – Frank Denis works at OpenDNS, and wrote DNSCrypt.