IMO

IMO - Acronym meaning In My Opinion.

Some others are WTF, GNU, HURD, PERL, and most other common expressions, and lots of Unix/Linux software that completely baffle any outsider to who dares peek into our realm use these too.
HURD is also thought to be the first doubly-recursive acronym.

Also the International Math Olympiad. This is a proof test that 6 person high school teams from around the world compete in.

The Internation Maritime Organization is the only UN agency based in England, its headquarters are London. Its primary remit is to promote maritime safety, and has also expanded to cover pollution issues, particularly oil spillage.