Mobile-friendly, minimal, private, and open-source. Works great on your phone as an add-to-homescreen web app, and there is no lock-in with pre-paid hosting.
Qloapps - Open Source System - Build Hotel Booking Website and Manage Online & On-Desk Bookings.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
FewohBee - Web based management tool for hotels, pensions, accommodations and vacation homes
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
BSI Hotel Booking System - BSI Hotel Booking System is a real time online hotel booking.
Google Tasks - to-do list app that integrates with Gmail and Google Calendar