Using Apple Remote Desktop, send the following UNIX Command to as many machines as you like to log them in. I suggest sending the command as the user you are logging in as. Note that this will only work if the machine is currently at the login screen.
osascript -e 'tell application "System Events" to keystroke "LOGIN_NAME"'; \
osascript -e 'tell application "System Events" to keystroke tab'; \
osascript -e 'tell application "System Events" to delay 0.5'; \
osascript -e 'tell application "System Events" to keystroke "PASSWORDHERE"'; \
osascript -e 'tell application "System Events" to delay 0.5'; \
osascript -e 'tell application "System Events" to keystroke return'
Replace LOGIN_NAME and PASSWORD with the proper values, of course…