Up until around 2004 or so, Hawaiian used to run a 717 between LNY and MKK...a distance of 27 miles.

Back in the 1990s, Delta and TWA used to have flights between MIA and FLL....a distance of 21 miles. In fact, Delta even used 757s and 767s on it (continuation from ATL).

Even further back, in the '70s or '80s, United flew 727s from SFO to OAK...a distance of 11 miles.

PacWings currently has one or two daily roundtrips between MKK and LUP (the cargo carrier I fly for also does a MKK-LUP run every morning)...a distance of 9 miles.