Speedbar is an abbreviated everything browser. If you can conceptualize browsable information into 20 columns, then Speedbar can display it for you.
Speedbar comes with several interfaces, with the primary one being for files and tags. It can also display buffer lists, RMAIL boxes, and Info nodes. Combined with CEDET, it also provides project tracking, smart completion lists, Class browsing, Connected graph browsing.
You can use speedbar to write your own browsers. Some external tools include ERC (Irc client), xslt-process, MH-E (mail reader), and ECB, a code browser which embeds Speebar.
Speedbar has been a part of Emacs for quite a while. The CEDET version of speedbar (as of this writing, Jul 08) is nearly identical to the version in Emacs 22.
For more details, read the see Speedbar Manual.