The powerful (Guide Number 110' @ ISO 100) yet affordable SF-3000 Digital Slave Flash from Vivitar is a sturdy unit with selectable firing and built-in digital slave circuitry. Plus, it features a switch on the back which allows you to select between the two different triggering modes, making it compatible with any camera on the market.
Ordinary slave units do not work with many digital cameras because these cameras use a very rapid set of pre-flashes to set the white balance of the camera's image sensor chip but not the exposure. The slaves will typically fire on the pre-flash it senses, while the digital camera captures the image on the last flash. Thus, the extra light from the slave does not show up in the digital camera photo. The SF-3000 flash units are designed to fire on a flash, sensed by their built-in optical sensor and circuitry, so the effect of the flash is not lost.