Personalmente utilizzo SuperSaaS (http://www.supersaas.it), ottima agenda nonché sistema di prenotazione online. Erogato come servizio web, con moltissime funzionalità e altamente personalizzabile.

If you’d like a tool for managing your time and projects, you can use this application inspired by David Allen’s GTD:

http://www.Gtdagenda.com

You can use it to manage and prioritize your goals, projects and tasks, set next actions and contexts, use checklists, schedules and a calendar.
A mobile version is available too.