TeXstudio, previously TexMakerX, is a fork of the LaTeX IDE Texmaker and gives you an environment where you can easily create and manage LaTeX documents.
It provides modern writing support, like interactive spell checking, code folding and syntax highlighting. Also it serves as a starting point from where you can easily run all necessary LaTeX tools. (you can see more features below)
You can run it on Windows, Unix/Linux, BSD and MacOSX systems and modify it if you want, since it is licensed under the GPL.