If you need to sign a legal document while away from your home or office you can use free web app SignNow to upload a PDF of the document, sign the document electronically and email it to wherever it needs to go, all without having to pay or register with SignNow.
Additional options include the ability to add text and/or sticky notes and to invite multiple signers. You can optionally register with SignNow to keep your signature on file and there is a beta premium version that adds identity protection tools and the ability to do real-time online signatures. Don't worry, the free version uses standard 256-bit encryption and when you receive the email with the PDF of the signed document, the email text includes the exact time the document was signed in addition to the IP address of the signer, to ensure the correct person actually signed the document.
Adding a signature is pretty straightforward. You can type your signature or use a mouse, trackpad or touchscreen input. If you already have a image file of your signature you can upload that as well.
From my examination of SignNow it appears to be a solid web app and I recommend it for freelancers and anyone else who has to deal with a lot of paperwork. I like that the document is emailed to me and if I tell SignNow to add a note to the email body identifying this document as "2011 tax forms", I can easily find the document in Gmail.