Of course, I use urxvt. It has support for changing fonts on the fly with escape sequences, so I put together a little script to make it easy. Having a tiny font can be pretty useful if you're watching log files go by or something is compiling and you just want to keep an eye ...

#!/bin/sh
# switch fonts on the fly in urxvt

# my default font
font="xft:DejaVu Sans Mono-10:antialias=true:hinting=true"

case $1 in
 small) font="7x13";;
 tiny) font="5x8";;
 # some crazy fonts I made out of my handwriting
 rupa) font="xft:rupa:style=Regular";;
 r) font="xft:rupa clean:style=Regular";;
esac

printf '\33]50;%s\007' "$font"