Increment-and-Decrement-number is a project mainly written in Vim Script, it's free.
Increment or decrement the number which is under your cursor
This is a mirror of http://www.vim.org/scripts/script.php?script_id=1882
In the command mode, move your cursor on the number which you want to change. Then use SHIFT and + to increment the number or SHIFT and - to decrement the number.