For various reasons I need to exclude some categories from being posted in so I exclude them in that particular page template. (step1.php for instance) To me it is always a hassle to go and lookup a category ID so I threw together a new page template that will dump id, category name and category slug for all available categories.
Installation is very easy, just copy the file into your CP directory. Make a new page and choose 'dump categories' from the template dropdown (on the right).
Name your page, save it and exclude it from the pages (in CP settings exclude pages) and that's it.
I just made a bookmark and open the page whenever I need to see category id's.
Hope this helps anyone else. If you have more categories than 1000 you should change the sourcecode of this page template to reflect that.
Maybe David can make another cp setting to exclude categories in an upcoming release? I noticed many people want to exclude some categories from being posted into.