If you're short on cash, opt for a used Vivitar 285HV or Nikon SB-24, SB-25, SB-26, or SB-28. They don't have iTTL, but do offer great manual control, which allows for lots of creative flexibility. They are also much cheaper than the newer speedlights.
Make sure to use the flash off-camera and read strobist.com for lighting suggestions.