United has introduced Star Alliance upgrade awards — upgrades on most of their Star Alliance partners confirmed by redeeming miles. These has been available on other Star Alliance carriers for several months, United has just introduced them, but it’s notable that only Star Alliance offers this quite so broadly.
United miles can be used for upgrades on ANA, Asiana, Austrian, LOT, Lufthansa, TAP Portugal and Thai Airways. (United miles could previously be used for upgrades on Lufthansa, but those were space available rather than confirmed.)
Currently there are no upgrade options for other Star carriers such as Singapore, Air Canada, bmi, Swiss, Spanair, SAS, South African Airways, etc. Hopefully they’ll come in the future.
Unfortunately you need to be buying nearly a full fare ticket in order to use this option. Y and B coach fares can be upgraded to business, and C and D business fares can be upgraded to first.