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I like to track my expenses in Org tables. For example:

#+tblname: daily-expenses
| Expense | Category |
|---------+----------|
|       2 | Food     |
|       1 | House    |
|      10 | Clothes  |
|       3 | Food     |
|       5 | House    |

At the of the month, I want to see how much was spent in each category. I wrote the following Babel function to calculate this:

#+name: group-categories
#+begin_src emacs-lisp :var table=daily-expenses :colnames nil
  (defun category-sums (catcol valcol)
    "Sum each of the categories in an Org table.

  \(fn CATEGORYCOLUMN VALUECOLUMN)"
    (let (gcats
          result)
      (dolist (row table)
        (let ((cat (nth (1- catcol) row)))
          (when (not (member cat gcats))
            (push cat gcats))))
      (dolist (gcat gcats)
        (let ((sum 0))
          (dolist (row table)
            (let ((val (nth (1- valcol) row))
                  (cat (nth (1- catcol) row)))
              (when (equal gcat cat)
                (cl-incf sum val))))
          (push (list gcat sum) result)))
      result))
  (category-sums 2 1)
#+end_src

Here is an example of the output:

#+results: group-categories
| Category | Expense |
|----------+---------|
| Food     |       5 |
| House    |       6 |
| Clothes  |      10 |

Dan Davison provided an alternate way of doing this using an R function:

#+begin_src R :var tab=daily-expenses :colnames yes
  attach(tab)
  aggregate(Expense ~ Category, FUN=sum)
#+end_src