Fastest way to find second (third...) highest/lowest value in vector or column

Use the partial argument of sort(). For the second highest value:

n <- length(x)
sort(x,partial=n-1)[n-1]

Slightly slower alternative, just for the records:

x <- c(12.45,34,4,0,-234,45.6,4)
max( x[x!=max(x)] )
min( x[x!=min(x)] )