how to emulate "insert ignore" and "on duplicate key update" (sql merge) with postgresql?

With PostgreSQL 9.5, this is now native functionality (like MySQL has had for several years):

INSERT ... ON CONFLICT DO NOTHING/UPDATE ("UPSERT")

9.5 brings support for "UPSERT" operations. INSERT is extended to accept an ON CONFLICT DO UPDATE/IGNORE clause. This clause specifies an alternative action to take in the event of a would-be duplicate violation.

...

Further example of new syntax:

INSERT INTO user_logins (username, logins)
VALUES ('Naomi',1),('James',1) 
ON CONFLICT (username)
DO UPDATE SET logins = user_logins.logins + EXCLUDED.logins;

Edit: in case you missed warren's answer, PG9.5 now has this natively; time to upgrade!

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Building on Bill Karwin's answer, to spell out what a rule based approach would look like (transferring from another schema in the same DB, and with a multi-column primary key):

CREATE RULE "my_table_on_duplicate_ignore" AS ON INSERT TO "my_table"
  WHERE EXISTS(SELECT 1 FROM my_table 
                WHERE (pk_col_1, pk_col_2)=(NEW.pk_col_1, NEW.pk_col_2))
  DO INSTEAD NOTHING;
INSERT INTO my_table SELECT * FROM another_schema.my_table WHERE some_cond;
DROP RULE "my_table_on_duplicate_ignore" ON "my_table";

Note: The rule applies to all INSERT operations until the rule is dropped, so not quite ad hoc.

For those of you that have Postgres 9.5 or higher, the new ON CONFLICT DO NOTHING syntax should work:

INSERT INTO target_table (field_one, field_two, field_three ) 
SELECT field_one, field_two, field_three
FROM source_table
ON CONFLICT (field_one) DO NOTHING;

For those of us who have an earlier version, this right join will work instead:

INSERT INTO target_table (field_one, field_two, field_three )
SELECT source_table.field_one, source_table.field_two, source_table.field_three
FROM source_table 
LEFT JOIN target_table ON source_table.field_one = target_table.field_one
WHERE target_table.field_one IS NULL;