Below you can find a list of multi-family complexes. We do not have information whether they have units for rent or not.
The (data in the) lists are not yet complete, come back soon to see updates.
For Acton you can also find the full list of existing affordable housing here. If you click on the development name, you can often find the contact details and website.
Condo vs Apartment
The biggest difference between a condo and an apartment is ownership. Condos are typically individually owned and rented out privately (they might be rented out via a realtor and/or on different websites), while an apartment is usually owned by a property management company that rents out and manages all of the units. They typically have a website and a central phone number for all rentals. You can find more information on the difference here.