There are many bookkeeping and accounting software sites for you to choose from. It all comes down to; what benefit you are looking to gain from it, whether it meets your business needs, whether it meets your budget, how much hand-holding you need, and how it fits into your daily productivity.
Quickbooks, Xero, and Freshbooks are bookkeeping and accounting sites that are tailored to accommodate any type of business, the interface isn't really friendly for someone looking for a quick overview of their properties.
Appfolio, Beaver, Buildium, Yardi, Rentroom, etc. are all property management sites that provide bookkeeping and accounting features tailored specifically for managing multiple units/properties, they also provide features such as maintenance request tracking and tenant screening.
If you're just looking to keep track of the financials of your units and nothing else I'd say Quickbooks, Xero, or Freshbooks. If you're looking for a more RE friendly site that provides benefits outside of knowing how much your units are making you, I'd say go with one of a property management site. I'd have to warn you though, some sites have minimum portfolio sizes, so YMMV.