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.
Kyle Eslick
Kyle Eslick is WordPress enthusiast who took his passion for WordPress to the next level back in 2007 by launching WPHacks.com as a place to share hacks and review WordPress-related products.
May 2012, and still a useful post. That's good value! 
Nice collection you have there. Spectrum was the one for me. Thanks!
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There’s actually a plugin made for 2.5 that will “reveal” the IDs again. You can get it here: http://wordpress.org/extend/plugins/reveal-ids-for-wp-admin-25/
I’m pretty sure the ID omission was a design decision, not an oversight. (Since the majority of users don’t need to know the IDs, and those who really need them can get them from the URLs, as you mentioned.)
^yeah, it was a bad idea then if it was a design choice… in my case, i was using a 3-column revolution theme and for the posts not to repeat in either columns, i needed to excluded categories by indicating the category id but since the 2.5 and 2.6 upgrade, i am having a hard time doing that now…
Bingo! I am a total n00b!
Thanks for this post.
This was great, easy to read and answered my question immediately! thx so much
I don’t understand why they made it so difficult to figure out the numbers.. anyways 
I’m good now.
Bingo!
I am a total n00b!
Thanks
for this post…