LXR is described as 'Cross Referencer, usually known as LXR, is a general-purpose source code indexer and cross-referencer that provides web-based browsing of source code, with links to the definition and usage of any identifier' and is an app. There are four alternatives to LXR for a variety of platforms, including Linux, Online / Web-based, Safari, Self-Hosted solutions and Sublime Text. The best alternative is Sourcegraph, which is both free and Open Source. Other great apps like LXR are DXR, OpenGrok and CodeNavigator.
DXR is a code search and navigation tool aimed at making sense of large projects like Firefox. It supports full-text and regex searches as well as structural queries like "Find all the callers of this function.
CodeNavigator is a developer's tool for browsing source code using CScope. It can help you to understand the structure of the code very easily and also include syntax highlighting support for many coding languages.