ShiftIt is extremely similar to Spectacle (I use both on different machines) and should be functional back to 10.6.

https://github.com/fikovnik/ShiftIt

I haven't tried Spectacle, but I'm happily running Divvy and Cinch on my 10.5 PPC Mac. :)

According to the info at MacUpdate, it's OS X 10.7 or later.

http://www.macupdate.com/app/mac/41147/spectacle

Spectacle should work on 10.6+ according to it's Info.plist: <key>LSMinimumSystemVersion</key> <string>10.6</string>

Sorry I'm not sure either, I know it works with latest OSX and the one before that.