[russellw]

Depends what you want. I like the Jabra units for their clean interface that just shows you who is calling and the quality of the audio as they use their own speaker but they do have to be installed.

There are a range of other units - some just a simple sunvisor mount with a built in speaker that require no installation through to FM units that use a frequency on the FM radio and often can make use of other phone capabilities like streaming music.