Is there an online dueling site

As of yet, no. But there are some programs that you can download and you can play other people Via IP Address or Via Hamachi.
I personally suggest KCVDS, which stands for Kaiba Corp Virtual Duelling System.
It's great and you can create any deck you want, the only bad thing is, you have to sub your own Life Points and such and apply the different effects.
Other than that, the program is absolutely brilliant and I highly recommend it