One of the disappointing changes in WordPress 2.5 was the removal of category ID numbers. I thought this was just a minor oversight, but since the 2.5.1 upgrade this problem still was not addressed.
If you are looking for a category number (this is most often needed for setting up the homepage of Magazine WordPress themes), here is a really easy way you can locate it until the WordPress team adds the category number display we all took for granted before:
1) Go to Manage > Categories.
2) Hover your mouse over the category that you need the ID of.
3) Look in the lower left corner of your web browser for the ID of that category.
Here is a screenshot example:
Hopefully WordPress will again display the category ID in one of the future updates, but thankfully this workaround is really easy.