According to TC: "mySupermarket, which has been around since 2006, claims to be the first comparison service that allows users to compare a cart of multiple items across retailers—in its case, groceries, across British supermarket chains Tesco, Sainsbury’s Ocado & ASDA."

In addition, top online grocery sites are:

Alice.com: consumer oriented personal shopper, could help find best deals for groceries including coupon and free shipping deals.
Amazon.com: offer a wide range of groceries, subscription for regularly delivery is free
MySupermarket.co.uk: UK-based comparison shopping
Freshdirect.com: mainly for New York metro, food from local farms and dairies, organic.
Netgrocer.com: has been in this category for a long time, mainly for health and beauty, organic food, babyfood, delivers to APO/FPO (military) addresses
AllSuperStore.com: offer standard grocery store items
ShopFoodEx.com: bulk order, large inventory, seasonal specials
ExpatExpress.com: specializing on oversea delivery
GoBioFood.com: organic and high quality food
Safeway.com: cover many US areas because of its physical presence