I created a work and now my company is using it with my permission. Who owns it?
This depends. If your work was created in the course of your employment, your employer will likely have copyright ownership. If you are an independent contractor and possibly signed a written agreement stating the work shall be "made for hire," your company will own the copyright if one of several conditions are met; for example, if your work is a part of a larger literary work, such as an article in a magazine or audiovisual work such as a screenplay under such an agreement, your company would own the copyright.
If you, the creator, have sold the entire copyright, the purchasing business or person would become the rightful copyright owner. However, if you simply permitted your employer to use your work that you have not created in the course of employment, you would still be entitled to copyright ownership of your work. The Copyright Act gives significant rights and powers to copyright holders, particularly when the copyright is registered with the U.S. Copyright Office, a step that confers certain additional legal protections on top of the right to bring an infringement action to court. See 17 U.S. Code § 106.