LaTeX is free software under the terms of the LaTeX Project Public License (LPPL). LaTeX is distributed through CTAN servers or comes as part of many easily installable and usable TeX distributions provided by the TeX User Group (TUG) or third parties. If you run into trouble, visit the help section.
Check your Linux distributions software source for a TeX distribution including LaTeX. You can also install TeX Live directly.
The MacTeX distribution contains everything you need, including a complete TeX system with LaTeX itself and editors to write documents.
If you're new to TeX and LaTeX or just want an easy installation, get a full TeX distribution. The TeX Users Group (TUG) has a list of notable distributions that are entirely, or least primarily, free software.
LaTeX is not a stand-alone typesetting program in itself, but document preparation software that runs on top of Donald E. Knuth's TeX typesetting system. TeX distributions usually bundle together all the parts needed for a working TeX system and they generally add to this both configuration and maintenance utilities. Nowadays LaTeX, and many of the packages built on it, form an important component of any major TeX distribution.
The LaTeX team cannot guarantee that TeX distributions, even recent ones, contain the most recent version of LaTeX. It may happen that you need a more recent LaTeX than the one that your favourite TeX distribution carries, e.g., in order to get a particular bug fix. In that case you will need to fetch LaTeX from CTAN and install it on top of your distribution. See below for details.
You can obtain LaTeX from CTAN, which is the primary source of distribution for LaTeX. In order for your downloaded LaTeX to be of any use, you have to obtain and set up a TeX system first. You can either install a TeX distribution (see above) or get a TeX system from CTAN. If you use a TeX distribution then it will include a version of LaTeX so this will probably make things easier for you; but you may have a reason not to do this.
LaTeX Subversion Repository
The LaTeX sources are kept in a subversion repository. Anyone can access it and download them, but submission is restricted to team members. The Subversion repository is located at https://latex-project.org/svnroot/latex2e-public/. You may check out a working copy of the code with the Subversion front-end of your choice or the command line statements:
svn co https://latex-project.org/svnroot/latex2e-public/ latex2e-svn
Ulrik Vieth has collected historic versions of LaTeX from LaTeX 2.0 for TeX 1.0 (released on 11 December 1983) onwards. You can find them at ftp://ftp.tug.org/historic/macros/latex-saildart/. There might even be some earlier versions. All files have been pulled from the SAILDART archive site at http://z.baumgart.org/ (no longer online) which was based on archive tapes from SAIL at Stanford.
More historic material can be found at ftp://ftp.tug.org/historic.