Developing mobile web is quite easier now with jQuery Mobile.
You can start developing mobile web applications using jQuery. To learn more visit http://jquerymobile.com/
Another good plugin prior to jQuery Mobile was JQTouch - http://www.jqtouch.com/
Yeah jQuery Mobile is a very good framework for building mobile site, only thing is, it is optimized for touch devices only, so using it means you’ll have to create a version of your mobile site for non-touch devices.