EasyBib

If you like writing citations as much as me you'll shout with joy over EasyBib, the free automatic bibliography and citation maker. First enter the source, a book,
journal, magazine of newspaper article, online database, website and some others (but not census, civil registration or other genealogical records.)  It checks a database to find the basic information. Then add detail by filling in more information in the boxes indicated. Last print or export the citation.

The free version produces results in MLA (Modern Language Association) format but you can buy access to APA and Chicago formats.

EasyBib has free iPad and iPhone versions which will also work by scanning the ISBN barcode.

There are others citation makers. See the list at 13 Types of Citation Generators.