How can I show a slideshow of pictures in multiple subfolders while keeping them in order? I have a folder that contains a number of sub folders that are full of pictures.  What I'd like to be able to do is start a slideshow from the parent folder that will show all the images in all the sub folders, however I would like the pictures to be shown in the order that they are in inside of each subfolder.

So, considering the following folders:

Parent folder
- Pictures 1
- More pictures
- Best pictures

I would like the slideshow to show all of the pictures in the 'Pictures 1' folder before moving on to 'More pictures' and so on.

Is this possible natively in OSX Lion?  I'm open to other tools is there's something reasonably priced.
While iPhoto may work for some, it doesn't come bundled with every Mac and it's not free. It also relies on a library.

The quickest, cheapest (free) and easiest way to view/slideshow a folder and its subfolders is using Sequential. Just drag it to your Applications folder, run it once so it shows up on your Open with menu, right click on a folder, choose Open with > Sequential and hit the 5 key (for a 5 second slideshow).

Check its preferences if you want it to always start in full screen and its menus for aditional hotkeys (show thumbnails bar or skip to previous/next subfolder, for example). Besides supporting images, it also supports PDFs and archives (.zip, .cbz, .rar, .cbr) and you can choose to display them sequentially (folder's default order) or randomly.