SyncPenguin vs PieSync
There's also another known service for synchronizations called PieSync. It supports two-way synchronizations and has similar to SyncPenguin goals. But there's an important restriction PieSync has: it only works with contacts, while SyncPenguin does not really care about what type of records to synchronize. Be it contacts, customers, meetings, orders, products or opportunities. As long as you can implement a data source app with the interface needed, SyncPenguin can synchronize it.
The lack of restrictions like that allows us to continue building an amazing library of pre-built synchronizations and get orders of custom synchronizations from our customers.