We're using FlightCircle, a cloud-based application that costs us $10/month (as EAA members). It was designed for FBOs so it has a lot of features we don't use, but the online scheduling works well for us. Scheduling, billing, maintenance and currency tracking all work well, and after an initially-steep learning curve it's easy to configure and use. I've talked with one of the principals at FlightCircle a couple of times about a particular issue that didn't make sense and and he update the program within a couple of days. Not saying they can do this in all cases, but they have been responsive to us, and knowledgeable about aviation operation. We're still waiting on a mobile app, though.
More information at https://www.eaa.org/eaa/pilots/flyin.../flight-circle
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