Google Calendar all day long.
Not fussy nor complicated, easily accessed, read, edited by either one person in the band or all of the band (latter is preferable). We are each responsible for putting in any of our 'unavailable' dates, which means that if any one of us gets offered a gig, you simply look at the calendar, if it isn't booked out or someone unavailable then that person takes the gig and puts it in the calendar (along with time/fee/contact name and number for the venue)... simple as!
You automatically get sent emails of updates that are put in the calendar, one person puts a booking in and everyone gets an email... up to the individual if they read it or not but it is there. You can set automatic reminders for weeks/days/hours before the gig, which is handy to remind you to send out posters.