dtrt-indent

An Emacs minor mode that guesses the indentation offset originally used for creating source code files and transparently adjusts the corresponding settings in Emacs, making it more convenient to edit foreign files.

You can install dtrt-indent from MELPA.

To activate it, M-x customize-variable dtrt-indent-global-mode, and turn on "Dtrt Indent Global Mode". See dtrt-indent.el for full documentation.

Related work

guess-style is similar to dtrt-indent, but only supports cc-mode, and only guesses offsets of 2, 4 and 8.

vim-sleuth provides similar functionality for Vim.