This code from Scripterlative uses JavaScript to change the behaviour of the enter button. Really handy if you’re building forms where your users may hit ‘enter’ to go to the next field, and accidentally submit the form before they’ve finished filling everything in.

Grab it here:
http://scripterlative.com/files/entertotab.htm

[N.B.: This code is provided by the author as 'GratuityWare' - that is, if you want to use it permanently you need to donate an amount of your choice to the author.]