I don't know of anything that gives full spreadsheet functionality. However, pg_access can simplify the manipulation of data by hand. Pg_access is an incomplete work-in-progress, adding capability to it would undoubtably be welcomed by the PG folks.
The author's been too busy to work on it recently, but an interested user/hacker has added some new capabilities to it. There was some discussion about it a couple of weeks ago, but I had a horrible case of the flu and quit monitoring the pg_hackers group. You can find archives of the mailing lists at www.postgresql.org if you're interested.
