Brianna’s Zesty French Dressing [1] is almost creamy enough to pass for an herbed mayonnaise. It is slightly spicy, but not overly vinegary. It is more runny than mayo, so be sure to figure that in when substituting.
[2]Brianna's Zesty French
Beware! Brianna’s labels often picture foods that are not actually contained within the dressing. The most famous kerfuffle involves their Dijon Honey Mustard [3] – which not only pictures an avocado, but is the exact same shade of green. There are no avocados in the Honey Mustard.
I wouldn’t use it in a chocolate-mayonnaise cake*, but in a case where you’re using mayo and don’t mind some spice and flavor, it works out quite well.
As of 6/2009 Brianna’s is available on Amazon [4].
*Yes, chocolate-mayonnaise cake. If you haven’t tried it, and you can have both chocolate and mayonnaise I recommend it.