Nassau County Police are encrypted and are no longer on their old UHF Frequencies.
There is a thread for ProVoice in the city, but you should be able to pickup some notes about what you might expect to notice in Nassau county:
http://forums.radioreference.com/ne...orum/324779-best-provoice-picks-nyc-area.html
As far as Nassau Monitoring goes, you can still monitor Nassau Fire, Nassau OEM, NYS Police, LIRR and MTA Police. Aviation and Marine are very active in this area as well. If you are to the east of the county, you should have no issues monitoring Suffolk's Trunk System. NYPD and FDNY should strong from anywhere in the county as well. If you are up north, give some Westchester and CT monitoring a try. It is a little late in the year for it, but NYS Parks and DEC make for some good listening too.