Agreements vary from publisher to publisher which is why it is important to read the document carefully before signing. Many publishers allow posting of articles on the Internet, but sometimes it is difficult to determine if it is permitted. As an example, the publishers below allow authors to post articles but with certain restrictions.
SHERPA/RoMEO is an excellent source for publishers' archiving policies and often includes links to the policy on the publisher's website. They also provide a list of publishers that allow authors to use the publisher PDF.
Increasingly, traditional publishers are offering authors the option of open access to their works for a fee. Examples are provided below and SHERPA/RoMEO provides an extensive list of publishers offering a paid open access option.