(example and www have asterisks next to them because "in addition to the above strings, ICANN will reserve translations of the terms “test” and “example” in multiple languages. The remainder of the strings are reserved only in the form included above.")
.mail would be excellent, really. All we need is $185,000, a stomach for paperwork and an established company looking to expand into the registry business.
String reviews (concerning the applied-for gTLD string). String reviews include a determination that the applied-for gTLD string is not likely to cause security or stability problems in the DNS, including problems caused by similarity to existing TLDs or reserved names.
localhost would be useless as it's in the hosts file by default. *.localdomain might be interesting. It's around, but I don't think it's the default (I don't know anyone that even uses it though).
EDIT: There's a list of reserved TLDs that won't be allowed to be registered. It includes localhost (but not mail)!
AFRINIC IANA-SERVERS NRO ALAC ICANN RFC-EDITOR APNIC IESG RIPE ARIN IETF ROOT-SERVERS ASO INTERNIC RSSAC CCNSO INVALID SSAC EXAMPLE* IRTF TEST* GAC ISTF TLD GNSO LACNIC WHOIS GTLD-SERVERS LOCAL WWW IAB LOCALHOST IANA NIC
(example and www have asterisks next to them because "in addition to the above strings, ICANN will reserve translations of the terms “test” and “example” in multiple languages. The remainder of the strings are reserved only in the form included above.")