There are a few sites and I think by years end will be more. But, employees may not be able to directly tap this new market yet - yet. But, if their employer agrees - the employer has to put out financial results - the employer could do it on the employees behalf. Right now, since this is such a young industry - the investors in these shares want high volume.
But, it is making a market (meaning providing liquidity for these shares).
Facebook made news last year for doing this - selling some employee shares - you can google that - try facebook and secondary market - also try http://www.secondmarket.com/
The NASDAQ also create a secondary market but I think you have to trade $100MM or more.