The  Key West Department of Transportation operates several bus and shuttle lines.  There are six color-coded routes that circulate around the ctiy, and two additional routes that travel between Key West & Marathon.  Maps and timetables are available online http://www.keywestcity.com/egov/docum...  

Note: it runs only every 60-90 minutes so you have to know the schedule, but if you need to get across the island or even to the airport ($2 versus $20) this is a good option

Fares can be purchased as you board the bus, at a variety of rates:

Key West Routes                                Lower Keys Routes 

$2.00 for a single fare.                      $2.00 for a single fare.
$8.00 for a seven-day pass.            $12.50 for a seven-day pass.

Children under five ride free.  Exact change is required.

If you have a bicycle, it can be secured to the bike rack on the front of the bus (Applies only to city routes). 

In addition to the bus system, taxi service is readily available.