How to record audio without connecting the mic to the camera?

I'm filming with a Nikon D90, and I can't input a shotgun mic into the camera directly, what is an alternative that I can do?
Is there a device that can plug into a laptop to save the audio directly? I have no problem syncing audio on to stripped footage.

I'm amused by your thought that sync will be no problem. Have you measured the timing clock in your laptop? I didn't think so. Rather than a laptop i strongly suggest you use a made for purpose device. Look at these options
http://www.getolympus.com/us/en/audio/digital-recorders.html