I've been using the Great Circle Mapper for the past year. Very accurate in my experience.

Here's what it returned for the examples posted to this thread.

ORD (41°58'43"N 87°54'17"W) SNA (33°40'32"N 117°52'06"W) 1726 mi
ORD (41°58'43"N 87°54'17"W) SFO (37°37'08"N 122°22'30"W) 1846 mi
BOS (42°21'52"N 71°00'19"W) SFO (37°37'08"N 122°22'30"W) 2704 mi
BOS (42°21'52"N 71°00'19"W) LAX (33°56'33"N 118°24'29"W) 2611 mi
BKK (13°54'45"N 100°36'24"E) NRT (35°45'53"N 140°23'11"E) 2887 mi