Gtags by Google
Server-based tags serving for large codebases Clients in python and for emacs and vim
Gtags is a speed-up of TAGS/etags for large source bases. In particular, the TAGS file is loaded into memory by a server, which then provides a narrowed view of the TAGS file to a client, resulting in a certain amount of speed-up due to having a much narrowed view.
Gtags has 3 components, the indexing component (which depends on the etags program distributed by EMACS), the server (which loads the index into memory), and the client (which talks to the server via IP).