Generate a dynamic sequence within a query

I know I've seen posts on how to do this, but i can't seem to find them.

I've got a data set

A, B
A, C
A, D
[...]

and so on

and i'd like to be able to wite a query that would result in

1,A,B
2,A,C
3,A,D
[...]

PG version is 8.3.

Any ideas?

Thanks

Dave