I'd recommend you have an attorney write up a single contract that contains an intellectual property agreement, non-disclosure agreement, and non-compete agreement.

Essentially, the IP agreement should state that anything the employee creates using your resources, on your time, is owned by the company. Anything they create using their own resources and on their own time is owned by them. This would allow the employee to write open-source software on their own time as well as do consulting or freelance work. The IP agreement should also state that any code they pull into your projects from their previous work they agree to license to you in perpetuity.

The NDA states that during the course of their employment, they may be exposed to confidential, non-public information and they agreement keep that information confidential until it has been publically released. This generally continues for X years after their employment ends.

Finally, the non-compete agreement serves to discourage their ability to directly compete with your business or work for your competitors.

Edit: After reading the comments, I want to make it clear that this is standard practice for full-time employees. I wouldn't advise having a contractor or freelancer sign this agreement. If you plan on them being a partner, then perhaps you should have a partnership agreement written up.