If you're travelling to Reykjavik and don't intend to figure out how to drive there yourself these buses are very comfortable and very easy and very cheap to use.
Budget-friendly transport from Reykjavik City to Keflavik International Airport. Bus departs from Gray Line Bus Terminal, pick-up is available free of charge from Bus stop 6 Safnahúsið / Culture House and from Bus stop 13 Rauðarárstígur in Reykjavik City Centre. Pick-up from the bus stops starts 30 minutes before the departure from the terminal.