jForms is a niceforms implementation for jQuery, in half the filesize of niceforms and nicejforms.
I've also made a number of improvements to the select box handling to make them behave more like regular select boxes, you can use your arrow keys to navigate up and down the list (without the browser window also moving, you can type with up to a second between key presses to search the box (ie. if you type in 'an' it will select the first item in the list beginning with 'an'), if the box has more than listSize entries it will be truncated and a scroll bar will be show (handy for long lists) and if there isn't enough room below the box to show the list, it will pop upwards instead (just like the real thing). You can also use the enter key to select the currently highlighted item and to open and close the list.