I can’t speak for the veracity of this list, but here are all the buildings proposed for Boston, over 300 feet tall.
W Hotel / Residences
Jacob Wirth’s lot
Nashua Street / North Station
Gateway Center (this isn’t the already built Gateway Center)
Filene’s (not even proposed, yet)
Suffolk Dorm (as if!)
South Station Tower
Mass Pike Towers
A great site to visit is emporis.com – they track all the high-rises proposed, under construction, or completed, in each city, all around the world. With photos! Awesome site.
The Boston Redevelopment Authority also tracks every new development proposed and/or approved for the city, although I’ve found the information isn’t always the most up-to-date or complete.