BusinessFormTemplate.com and Vertex42.com each offer several sample sign-up sheets. Forms are available for various purposes, and a small photo of each form is viewable prior to downloading and printing.
BusinessFormTemplate.com provides sample sign-up sheets for business needs such as mailing lists, as well as options such as potluck, tournament and general sign-up sheets that are adaptable for many uses.
Vertex42.com also offers a generic sign-up sheet for both business and personal uses that is available for download in Excel, Word, PDF and other document formats. This site also features dedicated sign-up sheets intended for use with potlucks and volunteering needs.