New version available check the next post.
This is the first plugin I created, using the spinbox plugin for jQuery, and wrapping some PL/SQL around it. It's very comparable to Patrick Wolf's mask example.
It's a very simple plugin, only rendering the field with extra up and down buttons at the end of the field, by clicking it adds or subtracts the "stepsize" parameter set in the settings. You can also set minimum and maximum parameters if you want to restrict the numbers (client side only ofcourse).
The demo: http://apex.oracle.com/pls/apex/f?p=44781:3
The download: http://www.apex-plugin.com/oracle-apex-plugins/item-plugin/spinbox.html
Have fun with it.