Sign Up Genius is a fantastic free site (with possible upgrade options for $) for creating sign ups -- think of the paper lists that get passed around for signing up for committees, food for potlucks, who's bringing what to parties/get-togethers, and contact lists.
An amazing tool that gets used exclusively at our school, pto's, classes, parent groups, theatre signups, sport teams, etc. The creation of a signup lets you: put in general info & a description, select a visual theme (they have tons or you can upload your own picture), create date and time slots, create named slots (will be a chaperone, provide a salad, sign up for a ticket taking slot, etc.), apply rules (sign up person must include email address and phone number), option for comments, and ability to communicate with all signup people via email or text. Very powerful and easy to figure out. I personally love the ability to copy an existing signup and modify it. A url is generated so it can be sent to mobile phones and signs up done there.
Here are some of the ways I have created and used:
1) Dinner signups on assigned days for families with illnesses/death with listed food item
2) Sign up for ticket taking at theatre production on multiple days and multiple time slots
3) Sign up for 15 different committees for band parent organization
4) Sign up for chaperone slots for scouting events, school trips,
5) Student sign up slots for volunteer opportunities and hours
6) Sign up for multiple offerings of CPR classes for our church
Fantastically powerful tool that people can use to sign up AND they are not required to create an account to sign up for anything (but creators do have to have an account) Notification settings can be set for the creator also (notify me when someone signs up, changes slots, cancels, etc.)