The most famous open source contract for freelancers is Andy Clarke’s “Contract Killer”, now at its third edition (and also available in Spanish!). This is a great plain-English agreement that sets a straightforward way to agree with your client. This contract is targeted to web projects but can be adapted to any kind of freelancing work.
Another type of document that freelancers sign is the independent contractor agreement (sometimes known as work-for-hire). This is a document usually provided by the client and for this reason very unfavorable to the freelancer. The linked example is a fair standard that you can adopt, or use as a benchmark against whatever they asked you to sign.
If you are a freelance writer, definitely check out this Work-for-hire Freelance Writing Agreement and Freelance Writer Assignment.
Repeated engagements with the same client? Consider switching to a Retainer!
Any important documents we missed? Let us know in the comments!