Caveat emptor: I've never used this tool myself. (Though perhaps I'll start...)
From long experience I've learned that I can put "emacs" + anything into google and find it. (Literally anything! Some people have waayyy too much free time on their hands...)
In this case, "emacs class browser" turns up EBROWSE, which might be what you are looking for.
In terms of a "left panel", emacs supports splitting a single window up vertically (split-window-vertically) or horizontally (split-window-horizontally) however you like. Or using multiple windows if you prefer (make-frame-command).
In terms of more typical IDE features, emacs supports font-locking (colorizing), make integration (jump to error), gdb integration (debugging), language-based auto-indent, TAGS [etags] (jump to tag definition), diff'ing two files (or file against revision), checkin/checkout to/from source code revision systems, spell-checking, etc. (I'm sure other stackoverflow entries have highlighted emacs's many features. Or try slashdot. Definitely check out the "complete" function! (require 'completion) And perhaps "align-regexp" too!)