Roman Republic

Railroad

Royal Air Force, IATA airline designator

Russian ruble

Rhesus factor

Reading Rainbow, an American children's television program

Road Rules, a reality series on MTV

Rachael Ray, an American television personality

Royal Road

Rio Rancho

Revised Romanization of Korean

Rational Recovery

Rural route

Road Runner (ISP)

Rear-engine, rear-wheel drive layout, in automotive terminology

Round-robin scheduling

The digraph rr

Rashtriya Rifles, Indian counter-insurgency force

Request-response, a synchronous messaging model commonly used in distributed computer systems

Reach Records, a Christian hip hop record label

Rolls-Royce

Regional Railways, a former sector of British Rail

Repurchase agreement rate (or repo rate; as in the Reuters Instrument Code US10YT=RR)

Relative risk in statistics and epidemiology

Rescue Rangers, a rescue organization headed by Chip 'n Dale and the TV series named for it

Respiratory rate as a vital sign

Riva Rocci, an abbreviation for blood pressure measured with a specific sphygmomanometer

Ronald Reagan, former President of the United States of America