FillAnyPDF.com is a website where you upload your PDF form and link to it so other people can fill it out and sign it online.
No software is needed. Any PDF form can be used, even if it's not "interactive", so you can get started right away. You can even invite a group to fill out your forms and track the results. Anyone that collects signatures or filled out forms will find FillAnyPDF.com to be a valuable time-saving resource.