One of the most used, but often not mentioned, methods for restructuring data is transposing matrices. This simply means switching the columns with rows and vice versa, via the t function:

> (m <- matrix(1:9, 3))
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8
[3,]    3    6    9
> t(m)
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
[3,]    7    8    9

Of course, this S3 method also works with data.frame, and actually, with any tabular object. For more advanced features, such as transposing a multi-dimensional table, take a look at the aperm function from the base package.