Texmaker is a free, modern and cross-platform LaTeX editor for linux, macosx and windows systems that integrates many tools needed to develop documents with LaTeX, in just one application.
Texmaker includes unicode support, spell checking, auto-completion, code folding and a built-in pdf viewer with synctex support and continuous view mode.
Texmaker is easy to use and to configure.
Texmaker is released under the GPL license .

(the structure, the document and the embedded pdf viewer side by side)

(two documents side by side)
Texmaker demo on YouTube :