oracle's first_value function for postgres?

Oracle has a very handy function called first_value, which can be used
to turn a set like this:

a   10
a   3
b   1
c   30
c   10
d   1
d   20

...into this:

a   10
b   1
c   30
d   1

Does postgres have something equivalent, or, even better, is there a
reasonable way to express this in standard SQL?