This was fun to put together, and reminded me of a lot of out of the way airports I haven't thought about for a long time! My total is 182: 134 domestic and 48 international. These are all on scheduled commercial flights.

Domestic
ABQ Albuquerque NM
ABR Aberdeen SD
AIA Alliance NE
ALM Alamogordo NM
ALS Alamosa CO
ANC Anchorage AK
ATL Atlanta GA
AUS Austin TX
BDL Hartford CT
BIS Bismarck ND
BNA Nashville TN
BOI Boise
BOS Boston
BTV Burlington VT
BWI Baltimore
BZN Bozeman
CAE Columbia SC
CDV Cordova AK
CHA Chattanooga TN
CHS Charleston SC
CLT Charlotte NC
CMH Columbus OH
CNM Carlsbad NM
COS Colorado Springs CO
CRW Charleston WV
CVG Cincinnatti
CVN Clovis NM
DAL Dallas TX
DAY Dayton OH
DCA Washington National
DEN Denver Stapleton
DEN Denver International
DFW Dallas/Fort Worth
DRO Durango CO
DTW Detroit
ELP El Paso
EUG Eugene
EWR Newark
FAI Fairbanks
FAR Fargo
FCA Kalispell
FLG Flagstaff
FMN Farmington NM
GEG Spokane
GSO Greensboro NC
GTF Great Falls MT
GUP Gallup NM
HLN Helena MT
HNL Honolulu
HOB Hobbs NM
HPV Princeville HI
IAD Washington Dulles
IAH Houston
ICT Wichita KS
IDA Idaho Falls ID
IND Indianapolis IN
ITO Hilo HI
JAC Jackson WY
JAX Jacksonville FL
JFK New York Kennedy
JNU Juneau
KOA Kona
KTN Ketchikan AK
LAM Los Alamos NM
LAN Lansing MI
LAS Las Vegas NV
LAX Los Angeles
LGA New York La Guardia
LIH Lihue HI
LIT Little Rock AR
LNK Lincoln NB
LRU Las Cruces NM
LWS Lewiston ID
MAF Midland TX
MCI Kansas City International
MCO Orlando
MEM Memphis
MFR Medford OR
MHT Manchester NH
MIA Miami
MKC Kansas City Municipal
MKE Milwaukee
MLI Noline IL
MOB Mobile AL
MSN Madison WI
MSO Missoula MT
MSP Minneapolis
MSY New Orleans
OAK Oakland
OGG Kahului
OKC Oklahoma City
OMA Omaha
ONT Ontario CA
ORD Chicago O'Hare
ORF Norfolk VA
PDX Portland OR
PHL Philadelphia
PHX Phoenix
PIH Pocatello ID
PIR Pierre SD
PIT Pittsburgh
PSC Pasco WA
PUB Pueblo CO
PUW Pullman WA
PWM Portland ME
RAP Rapid City SD
RDU Raleigh NC
RIC Richmond VA
RNO Reno NV
ROW Roswell NM
SAF Santa Fe NM
SAN San Diego CA
SAT San Antonio TX
SAV Savannah GA
SBP San Luis Obispo CA
SEA Seattle
SFO San Francisco
SGU St. George UT
SJC San Jose CA
SJU San Juan PR
SLC Salt Lake City UT
SMF Sacramento CA
SMX Santa Maria CA
SNA Santa Ana CA
SPI Springfield IL
STL St. Louis MO
SUN Sun Valley ID
SVC Siver City NM
TLH Tallahassee FL
TPA Tampa FL
TSM Taos NM
TUS Tucson AZ
TVC Traverse City MI
YAK Yakutat AK

International
AKL Auckland NZ
BJX Leon MX
BKK Bangkok TA
BNE Brisbane
BOG Bogota
BRU Brussels
BUD Budapest
CDG Paris
CLO Cali
CPT Cape Town
CVM Ciudad Victoria
EDI Edinburgh
FCO Rome
FRA Frankfurt
GDL Guadalajara
GYE Guayaquil
HKG Hong Kong
HKT Phuket
JNB Johannesburg
KIX Osaka
LGW London Gatwick
LHR London Heathrow
MAD Madrid
MEX Mexico City
MID Merida MX
MTY Monterrey MX
MXP Milan
NRT Tokyo
ORY Paris Orly
PPG Pago Pago AS
PTY Panama City PA
PUJ Punta Cana DR
PVR Puerto Vallarta MX
SIN Singapore
SJO San Jose CR
TAM Tampico MX
TPE Taipei
TUA Tulcan EC
UIO Quito EC
VER Veracruz MX
WDH Windhoek NA
YOW Ottawa
YQB Quebec City
YUL Montreal
YVR Vancouver
YYZ Toronto
ZIH Zihuantenejo MX
ZRH Zurich