For most of people who start Marco photography, the best bet is to get  Nikon 105mm f/2.8G ED-IF AF-S VR Micro-Nikkor ($895). This lens replaces the Micro-Nikkor AF 105mm f/2.8 D, a dated but popular and highly-regarded lens.  Nikon 105mm f/2.8G ED-IF AF-S VR Micro-Nikkor has a min. focus distance of 1 feet resulting in a max. object magnification of 1:1 and a working distance of min. 6 inch. The filter size is 62 cm. This lens is extremely sharp and has a great bokeh (blurred background).  In addition, it has 4 f- stop vibration reduction.  Nikon 105mm f/2.8 is also a great portrait lens.

Nikon 60mm f/2.8G ED AF-S Micro-Nikkor Lens is also a good macro lens with a max. magnification ratio of 1:1.  when used in close-up work, at maximum magnification the subject distance to the front lens is only about 2 inch. Therefore, this lens is good for flowers.

For more affordable alternatives,

Tamron AF 90mm f/2.8 Di SP A/M 1:1 Macro Lens($469)

Tokina AF 100mm f/2.8 AT-X Pro D macro ($399)

Sigma AF 105mm f/2.8 EX macro DG

are all as sharp as Nikon macro lenses and all are more affordable than Nikon lenses.  Comparing with Nikon 105mm f/2.8G ED-IF, these lenses are not internal focusing and also there is no vibration reduction.  Two features are worth the extra $$$.

Another great and affordable macro lens which can also provide longer working distance is Sigma 150mm f/2.8 EX DG HSM APO HSM IF Macro Lens for Nikon SLR Cameras ($620).