All known neutral buildings are listed below, with detailed descriptions and screenshots (where possible).
Warehouse: This building is found scattered around the maps, and inside contains raw materials needed to build your base. Your Chinook/Trucks/Workers will travel to and fro from your base to these structures and collect the resources, adding a healthy amount of $$$'s to your account. Whether they slowly empty out over time is unknown, but all sides have secondary means of getting cash anyway.
Derrick: You're sure to remember this building from RA2, although we're not completely sure of its' use in Generals. One could guess it delivers a steady stream of cash to the owner, which is most likely.
Radio Tower: Never seen one of these before, so we're at a bit of a loss as to what it does. It could provide a secondary radar if your own goes down, although it has been suggested it could interfere with your enemies units and possibly give you control of their commands.
Buildings: Normal houses and such, but you can easily garrison your troops inside and halt any imcoming forces. The size of the building decides how many infantry you can squeeze in, so large office blocks should be garrisoned quickly to give you an advantage in city-based maps.
Hospital: Once again imported from RA2, it doesn't take a genius to realize this building will take your infantry in and heal them up ready to fight again. Expect to see a lot of them in city maps, but keep your soldiers fit and strong while battling in the remote mountains.
Workshop: Essentially a repair shop, you can send your tanks and such in to mend their battered armour and get them back to full power. This will certainly cut costs on having to build more tanks, so make sure you capture all of these buildings, and repair every tank before heading into a firefight - that last scrap of armour could decide the outcome of the battle.