Mac: PDFpen is a smart and simple application that helps you fill out and sign PDFs without printing anything, perfect for times you have to fill out a little paperwork. No more print, sign, scan, then email.
PDFpen (made by the people who developed the incredibly time-saving text substitution app TextExpander) is perfect for anyone who fills out a lot of paperwork, but it's just as handy for those one-off documents. There's not much to getting started with it: Download the app, open it, pick a PDF to fill out, then grab the pen or text tool and get to work.
PDFpen is available as a free demo, but all PDFs saved with the trial will be waterstamped. That's good enough for a lot of paperwork (it's the version I use), but if you need it waterstamp-free, the $US60 version removes the waterstamp. Thanks Matt and Jason!
PDFpen [Smile Software]