I am using Go Launcher Ex on my Samsung Galaxy Ace. And I am a happy user of it. Almost using their every app. Because, it gives you the best control of the way you want to customize it. And for free.Recently they have released a lock screen replacement, Go Locker.
WidgetLocker is also a powerful lock screen replacement, but that is not free. Go Locker is a good deal to try out for free. Comes with more themes and notification and shortcut features on your lock screen.
Or download Go Locker v 1.0 Beta 2 APK.