Different publishers have different restrictions on posting articles online, and it's important to understand your rights before signing an agreement. For example:
SHERPA/RoMEO provides information about publisher policies, as well as a list of publishers that allow authors to use the publisher PDF.
Some traditional publishers offer an open access option to authors in exchange for a fee. Examples of journals and fees are provided below and SHERPA/RoMEO provides an extensive list of publishers offering a paid open access option.