Why can't pgAdmin3 connect to Postgres? I added a new user to posgresql (amanda) which is the same as my linux system user. At the console, I can open psql just fine -- I don't need a password because I'm already authenticated as amanda. But when I try to set up pgAdmin III without a password, I get an error: 

An error has occurred:

01:18:47 PM: Error: Error connecting to the server: fe_sendauth: no password supplied


And if I provide my system password, I get a password error: 

An error has occurred:

01:15:30 PM: Error: Error connecting to the server: FATAL:  password authentication failed for user "amanda"
FATAL:  password authentication failed for user "amanda"


How do I get pgAdmin3 talking to postgres as amanda?

This is all @ localhost. I read this:

PgAdmin III - How to connect to database when password is empty?

but the answer there discusses trust authentication. My read of the auth methods section of the manual suggests that what I want is for pgAdmin3 to use Ident Auth.
On Unix, when psql is used without the -h option or a PGHOST variable set, it uses Unix local domain sockets, which corresponds to local in the TYPE column in pg_hba.conf and no ADDR column at all.

That differs from putting localhost in the host field of pgAdmin's connection dialog. This one corresponds to host in the TYPE column and 127.0.0.1 in the ADDR column or whatever localhost is resolved to by the TCP stack (with IPv6 it could be ::1 for instance).

In general, this is what is confusing when psql and pgAdmin connection attempts do not trigger the same rules in pg_hba.conf. If that's the case in this question, try to leave the host field in pgAdmin empty.

On top of that, Windows does not have Unix local domain sockets so psql on Windows defaults on localhost as if -h localhost was used.