I've used these and recommend them: Power Strip Liberator Plus Extension, they are available from a variety of places but I found a good price on ebay : http://stores.ebay.com/Great-Lakes-MercantileQuote from FuturesTrader71:
The way to get around this is to buy a 2-prong plug available at Radio Shack really cheap. It is a short cord (6 inches) that has 2 female outlets on the end of it. You can then plug your large transformers into it. For $2.99+tax, they can't be beat.
Here's the link:
http://www.radioshack.com/product/index.jsp?productId=2103297&cp
The description says 6' cord, but it is actually 6".
They provide a 6" pigtail so that a transformer doesnt take up multiple outlets and they also have a pass-thru feature so you can still use the outlet its plugged into