The authoritative list of all TLDs: http://www.iana.org/domains/root/db
The Root Zone Database represents the delegation details of top-level domains, including gTLDs such as .com, and country-code TLDs such as .uk.
You can get this as TXT, too (easier to parse): http://data.iana.org/TLD/tlds-alpha-by-domain.txt
Wikipedia also lists all TLDs, but it's not authoritative, of course: List of Internet top-level domains
The Mozilla Foundation maintains a Public Suffix List for effective top-level domains, i.e., those under which you can directly register names (for example .co.uk instead of .uk): http://publicsuffix.org/
