echo -e "\e[1;40m" ; clear ; while :; do echo $LINES $COLUMNS $(( $RANDOM % $COLUMNS)) $(( $RANDOM % 72 )) ;sleep 0.05; done|gawk '{ letters="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789@#$%^&*()"; c=$4; letter=substr(letters,c,1);a[$3]=0;for (x in a) {o=a[x];a[x]=a[x]+1; printf "\033[%s;%sH\033[2;32m%s",o,x,letter; printf ?"\033[%s;%sH\033[1;37m%s\033[0;0H",a[x],x,letter;if (a[x] >= $1) { a[x]=0; } }}'; reset
Instead of manually creating your list of characters, you could also use the jot command to auto generate the available ascii characters for you.

echo -e "\e[1;40m" ; clear ; characters=$( jot -c 94 33 | tr -d '\n' ) ; while :; do echo $LINES $COLUMNS $(( $RANDOM % $COLUMNS)) $(( $RANDOM % 72 )) $characters ;sleep 0.05; done|gawk '{ letters=$5; c=$4; letter=substr(letters,c,1);a[$3]=0;for (x in a) {o=a[x];a[x]=a[x]+1; printf "\033[%s;%sH\033[2;32m%s",o,x,letter; printf "\033[%s;%sH\033[1;37m%s\033[0;0H",a[x],x,letter;if (a[x] >= $1) { a[x]=0; } }}'
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Terminal Screen Saver

August 11th, 2011
Full terminal color gradient effects. The modulus operations keep the colors cycling a bit and makes sure it repeats itself after producing some horizontal lines. Probably could be reworked to be a bit better, this is an exercise left to the hacker.

clear; j=0;a=1;x=1;y=1;xd=1;yd=1;while true;do for i in {1..2000} ; do if [[ $x == $LINES || $x == 0 ]]; then xd=$(( $xd *-1 )) ; fi ; if [[ $y == $COLUMNS || $y == 0 ]]; then yd=$(( $yd * -1 )) ; fi ; x=$(( $x + $xd )); y=$(( $y + $yd )); printf "\33[%s;%sH\33[48;5;%sm . \33[0m" $x $y $(( $a % 8 + 16 + $j % 223 )) ;a=$(( $a + 1 )) ; done ; x=$(( x%$COLUMNS + 1 )) ; j=$(( $j + 8 )) ;done
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Rainbow Cursor Worm

August 11th, 2011
Cursor bounces from walls of terminal trailing a rainbow pattern behind.

clear; a=1;x=1;y=1;xd=1;yd=1;while true;do if [[ $x == $LINES || $x == 0 ]]; then xd=$(( $xd *-1 )) ; fi ; if [[ $y == $COLUMNS || $y == 0 ]]; then yd=$(( $yd * -1 )) ; fi ; x=$(( $x + $xd )); y=$(( $y + $yd )); printf "\33[%s;%sH" $x $y $(($a%199+16)) ;a=$(( $a + 1 )) ; sleep 0.001 ;done
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Cursor Bounce

August 11th, 2011
Cursor will bounce off the walls of your terminal. Try resizing your terminal while its in the middle of the terminal.

clear; x=1;y=1;xd=1;yd=1;while true;do if [[ $x == $LINES || $x == 0 ]]; then xd=$(( $xd *-1 )) ; fi ; if [[ $y == $COLUMNS || $y == 0 ]]; then yd=$(( $yd * -1 )) ; fi ; x=$(( $x + $xd )); y=$(( $y + $yd )); printf "\33[%s;%sH" $x $y; sleep 0.02 ;done