I've personally been pretty happy with SalesForce's Desk.com, it offers a great interface, one agent is free and you have the option of flex agents ($1/hr) inst

I've personally been pretty happy with SalesForce's Desk.com, it offers a great interface, one agent is free and you have the option of flex agents ($1/hr) instead of paying for a full seat every month when you just need an occasional helper.