These days almost all forms are distributed online as PDFs. However, when you need to sign one, you normally need to print it and scan it back in for an email, or even worse, use a fax. Luckily, there are apps for that.
PDF Signer allows you to fill out and sign PDF forms. You can enter text in any pre-defined text boxes the form might have, but if the form isn't that advanced, you can enter text in text boxes of your own creation anywhere on the PDF using the "Add Text" tool.
Once you've filled in all the relevant information, now comes the tricky bit -- getting the form signed. Unfortunately, there is no direct signing support within PDF Signer. Instead you have to overlay an image file of your signature, which you've created elsewhere, on the form. Once you've selected the appropriate image, you can resize it to fit the box and save or print the PDF.
If you've got an image file of your signature handy, PDF signer makes short work of signing PDF forms. But given that you can enter text on PDFs using OS X's Preview, I would have liked to see some sort of tablet or trackpad support for creating a signature, not just inserting an image overlay.PDF Signer is available from the Mac App Store for $9.99.