Boulder has a fairly good public bus transportation system that has some good main routes that run quite frequent for public buses.
It is run by RTD Regional Transportation District the main bus lines are the, SKIP, HOP JUMP, DASH, BOUND and STAMPEDE the main buses come about every 15 minutes during peak hours, Monday-Friday.
Other routes, such as the 201, 203, 204, 205, 206, 208 and 209 run every 15 to 30 minutes.
RTD Regional routes run from boulder to Denver, DIA and Longmont, these main regional routes run quite frequently and are usually fairly easy to get on.
Be sure to have exact change because RTD doe not provide change.
There is a couple of taxi and airport shuttle services that serve the Boulder area also.