A somewhat sensible use of <<-

gistfile1.r

## Captures a variable 'n' in the local scope of the returned
## function.  When you run that function it increments the counter and
## prints its current value.
make.counter <- function(start=0) {
  n <- start
  function() {
    n <<- n + 1
    message(sprintf("counter value is %d", n))
  }
}

f <- make.counter()
g <- make.counter()

f() # 1
f() # 2
g() # 1 -- independent of f

## Better, using S3 classes:

make.counter <- function(start=0) {
  n <- start
  f <- function() {
    n <<- n + 1
  }
  class(f) <- "counter"
  f
}

print.counter <- function(object, ...) {
  message(sprintf("Counter value is %d", environment(f)$n))
}

f <- make.counter()
f()
print(f)