I was looking for a good introduction to Emacs for some friends and stumbled upon the truly excellent Guided Tour of Emacs on gnu.org.  And unsurprisingly it contained a few useful minor modes of which I had never heard.

icomplete-mode

icomplete-mode shows completions in the minibuffer as you type.  If you’re too impatient to hit tab, then this is the minor mode for you.

iswitchb-mode

This global minor mode solves an inconvenience that had always bothered me.  Typically, to see a list of buffers without resorting to the mouse, one has to C-x b TAB to see the list of buffers, or C-x C-b and then toggle over to the other window (Ctl-o) to use dired to select a buffer. Too many keystrokes.

With iswitchb-mode turned on, C-x b shows a list of avaialble buffers in the mini buffer, narrowing them down as you type.  A much easier way to jump between dozens (hundreds?) of open files.

Update: Looks like iswitchb-mode has been replaced with the far superior ido-mode as of Emacs 22.