Print a sinusoidal wave (vertically)
C, 74 73 70 69 67 characters

67 character solution with many good ideas from @ugoren &amp; others:

i;main(j){main(poll(printf("%*c|\n",j=21+sin(i++*.1)*20,0),0,50));}


69 character solution with while loop instead of recursion:

i;main(j){while(printf("%*c|\n",j=21+sin(i++*.1)*20,0))poll(0,0,50);}


Approaching perl territory. :)