I have to shoot some indoor "on location" portaits in office buildings. The subjects are executives of companies. I'm going to use natural light if possible, but as a backup, I will use a Canon 550EX flash with my Canon 10D DSLR, along with various LumiQuest light modifiers and off-camera extensions. I'd like to have a slave flash or 2 as well for background fill/hair light. Can anyone recommend a good, battery-operated slave unit? I'd like it to be portable and (hopefully) relatively inexpensive (i.e., < $100). I know I can use another 550EX, but that's $300. I've seen some small slaves, (GN 25-30 or so), but I don't know if they will work. Thanks.
Try the Vivitar DF200. It has a respectable Guide number (92 ISO @ 100ft). It has both bounce and swivel, as well as 1/16, 1/8, 1/4 settings. It also has click settings for wide (28mm), medium (50mm), and zoom (105mm). All this for about $70-$80 street. And if that wasn't enough, it comes with a mini-flexpod and an under-camera flash bracket.
Alternately, if you can use about half the power and fewer features, then the SP-Systems Midi Slave DC Flash is about $25 street.