knoppix5's rhomb: http://www.commandlinefu.com/commands/view/14153

rhomb1.sh

L=$((LINES/2))

function f()
{
	for A in $(seq $L); do
		for B in $(seq $L); do
			[ $A -lt $B ] && C=' ' || C=x
			echo -n "$C "
		done
		echo
	done
}

X=$(f | tac; f)

paste <(echo "$X") <(echo "$X" | rev) | sed 's/\t //'

rhomb2.sh

S=$((LINES*2-3))
L=$(printf "x%${S}sx" "")

function g()
{
	for A in $(seq $((LINES/2))); do
		echo "$L"
		L=$(echo "$L" | sed 's/    \( \+\)/ x\1x /')
	done
}

g | tac
g