Mike Talbot has created a new class called AutoGrid which allows you to define Rows and Columns like an ordinary grid, but if you don’t catch all of the cases then it fills out the rest for you.
Source: Mike Talbot's Blog
I always find myself cursing Grids in Silverlight when I have to go back and insert something else into the layout after the fact: I always forget to move somethings Row and end up with a lot of obscured components. To compensate for this I’ve written a Grid subclass that can automatically layout its children and fill out the ColumnDefinition and RowDefinition for missing values when the children refer to rows and columns beyond the specified dimensions. In addition it will auto layout for multiple columns using sequential children, this is good if all you want is a 2 by n grid of label/editor cells.