If you have lost your password on Mac, here are a simple list on how to reset your password:
Mac OS X 10.7 - Lion
1. Boot into single user mode (press Command-S at power on)
2. Type fsck -fy
3. Type mount -uw /
4. Type launchctl load /System/Library/LaunchDaemons/com.apple.opendirectoryd.plist
5. Type ls /Users
6. Type dscl . -passwd /Users/username password, replacing username
with the targeted user and password with the desired password.
7. Reboot
Mac OS X 10.6 - Snow Leopard
1. Boot into single user mode (press Command-S at power on)
2. Type fsck -fy
3. Type mount -uw /
4. Type sh /etc/rc
5. Type ls /Users
6. Type passwd username
7. Reboot
Mac OS X 10.5 - Leopard
1. Boot into single user mode (press Command-S at power on)
2. Type fsck -fy
3. Type mount -uw /
4. Type launchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
5. Type ls /Users
6. Type dscl . -passwd /Users/username
7. Reboot
Mac OS X 10.4 - Tiger
1. Boot into single user mode (press Command-S at power on)
2. Type fsck -fy
3. Type mount -uw /
4. Type sh /etc/rc
5. Type ls /Users
6. Type passwd username
7. Reboot
Last Resort! WARNING! You may lose settings with this one!
1. Boot into single user mode (press Command-S at power on)
2. Type fsck -fy
3. Type mount -uw /
4. Type rm /var/db/.AppleSetupDone
5. Reboot
There it is. Quick and ugly. If there's any question, please, feel free to comment.