Below is an email sign up form for my email list using float labels implemented with floatlabels.js
Enter your name. Delete it. Type it again. Enter your email. Delete that. Retype. Enter a misformatted email. Now hit submit. Check out the error handling, etc. Have a blast with it.
For more info on the float label pattern, read this post. You can also find resources for your own implementation in iOS, Java, CSS, etc. here.