A list of the top-level domains by the Internet Assigned Numbers Authority (IANA) is maintained at the Root Zone Database.
[1] IANA also oversees the approval process for new proposed top-level domains for ICANN.
The IETF has submitted several requests for comments on TLDs that could be used to represent local devices and services.
[251] Generally, these non-standard domains are unreachable through the normal DNS resolution process and instead require clients to use some sort of transparent web proxy or gateway to access them In the case of alternative DNS roots, organizations or projects make use of the same mechanisms of the DNS but instead take on the role of ICANN in managing and administering an entirely separate root zone, thus having the ability to create new TLDs independently.
However, this doesn't make these domains any less isolated from the rest of the internet, though the ability for clients to resolve them theoretically only requires switching to a recursive DNS resolver that recognizes and serves records underneath the alternate root zone.