Mendeley is a "free reference manager and academic social network". It aims to work as a central hub for your research material. Known for its wide variety of features such as PDF highlighting/note taking and automatic extracting of metadata from PDF files.
Zotero is a "free, easy-to-use tool to help you collect, organize, cite and share your resource sources". Originally a Firefox extension, now also available as a standalone desktop program. Enables you to easily scrape bibliographic metadata from web pages.
This page is covers reference managers that are used with "what you see is what you get" type text processors such as OpenOffice and MS Word.
LaTeX on the other hand is an in depth document preparation system mostly used for complex scientific writing. LaTeX has its own page in the top menu.