Easy Contact Form is fantastic! My website had been around for a long time and has quite a large base of new customers as well as returning customer but since using this little lightbox have had a major increase in contact us forms inquiries. I'd say about 80% of which have turned into conversions (off course you have to have great customer service as well). The best part about it, it is FREE! Also I have had many customers complementing the form, saying it makes the site look very professional and trustworthy.
Have a look at my website to see the live version of it in action!