This is definitely high on my list of favorite websites now: Coovents shows you what happy hours are ongoing in New York at any given time.
One cool discovery via Coovents: $10 manicure + martini at Beauty Bar from 6-11pm, Monday to Friday. Judging from photos on the website, the place isn't super swank. But neither is my manicure place round the corner, and they don't spring for water much less free alchie. Must try this one!
PS: There are also Beauty Bars in San Francisco, Austin, Los Angeles and Las Vegas. Get the deets on their website.