PDFfiller is a simple and easy to use web app which lets you fill in pdf forms online and edit them to some extent. You could upload PDF files directly from your computer or by providing the web url. As you know, most of the PDF readers like Adobe Reader would allow you to read a PDF document but not edit it unless you spend money on their professional versions.
Hence a free (not free) utility like PDFfiller can save you the hassle of printing out PDF forms and then fill it. Instead you could just upload the document, fill it and then email or fax it.
The way PDFfiller works is that it converts the PDF document to an image and then allows you to edit it. The nice thing about this app is that it allows you to email, fax or print the filled form from within its interface.
Watch demo video below:
Update: Apparently the service is not free and asks users to pay at the last step (see comments). For a free altarenative tool you may want to check out PDFescape, PDFHammer and PDFVue (recommended).
Check out PDFfiller @ www.pdffiller.com