See also Work-product doctrine for how this applies specifically to attorneys.

Work product is something created by an employee that is considered to become the property of the employer -- for instance, a design, patentable process, recipe, etc. -- given certain conditions. It becomes trickier when the employment relationship is one of contractor-client. An independent contractor has different work product rules from an employee.

Example from Graphic Design

For instance, if I create a printed brochure for a client on my computer with my software as an independent contractor, he's entitled to the printed brochure itself, and probably the final file sent to the printer (usually a locked-down, non-editable PDF file) to produce the brochure, but he is not automatically entitled to the original digital working files (the InDesign publication files, the layered Photoshop files... the files where editing is performed.) without paying extra for them (a "buyout") unless he has a signed contract specifying otherwise. (A standard buyout figure is a 20% surcharge on the original job price.)

But if you are an employee who produces the work on the employer's computer with the employer's software in the employer's place of business, not only does he own all of the work and files, you might actually need his permission to, for example, show the work on your website as part of your portfolio... again, unless you have a signed agreement to the contrary, up front.