I take a lot of screenshots in OSX (⌘+shift+4) to share and for the longest time I just lived with the 2003-era super cool shadow:
I finally got tired of it and found the solution to turning it off is these two terminal commands:
defaults write com.apple.screencapture disable-shadow -bool true
killall SystemUIServer
Now my screenshots look like this:
If for some reason you would want to re-live 2003 you can re-enable the drop shadow with these commands:
defaults write com.apple.screencapture disable-shadow -bool false
killall SystemUIServer