This week I want to pay tribute to an open source project called the LXR Cross Referencer. LXR is a web tool that lets you browser the source code of a software project, navigating link by link based on included source files, functions or variables.
LXR can be downloaded from the project’s website  and applied to any software project.
The most popular instance of LXR is found on the project’s initial page  as an instance for the Linux Kernel. This site has a complete history of the Linux code since version 0.0.1 to latest stable version. Opening two windows of two different versions of a file, you can compare the code and see what’s been added or changed between the versions.
It’s very useful for finding where a function or a constant has been used, or to see in what header a function has been declared, defined and then used.
Note that all of the above can de done via command line tools like ctags or cscope alongside vim or emacs, with grep -r, diff and git. But the friendly part of lxr.linux.no is that everything is already on the site so you don’t need to download anything locally and you can use everything there as long as you have an Internet connection.