If you are looking for an option to allow your clients or potential clients to book time with you directly, I currently use TimeDriver (
www.timedriver.com), it is a pay for service but pretty cheap, like 29.00 per year. What like about timediver is that I can set up different blocks of time for different scenarios, so I have a time block for new client consultations, current client calls etc....You can integrate in your email or on your site, with a button graphic that takes to you to the page. You can see a sample by going to MyCalendar.ReneeShupe.com
A great free service is
Tungle.me and I have also used that, but when I switched TungleMe lacked a few features I really wanted.
Google calendar has now opened up appointment slots and both of these services integrate with Google Calendar, so I never have any back and forth with new potential clients or current clients.
In regards to diary management, I've just started to use
Mingly which is a plugin in for Google Gmail or Google Apps, you need to be running Chrome (not sure about about Firefox) to operate it, but so far I quite like it. It has a space for notes, follow up options (one time, every month etc) and best of all it's free. You would need to search in the Chrome Webstore to find Mingly, but it fits my needs so far. I then use
Evernote for the remaining portion of my diary management and project management.