Adobe's got a HUGE selection of actions on their site (some great, some not-so-great). That URL ATNO posted, I actually found it last night through Google (normally I just make my own) and downloaded two or three (hundred, heh) actions to play around with.
Actions are essentially a photoshop macro (the other type, not the photography kind, heh). It's a recording of a series of commands & mouse clicks in photoshop, that you can play back at a future date to reproduce a sequence of events.
I use these all the time, a good example of a simple action you can create is simply to resize images to produce thumbnails. If you go out and take 50 pictures with your digital camera, it can be a pain in the butt to resize all those manually.
So, here's what I do.
1. Copy all the pics from the camera/memory card to the PC and put just those photos in a single directory with nothing else.
2. Copy that directory to a new name (the work directory), so that I have two identical directories with different names.
3. Open up a single image in the work directory. Start recording an action.
4. Image, Resize, enter my sizes, click ok
5. CTRL+S (save image)
6. CTRL+F4 (close image)
7. Quit recording the action
8. File -> Automate -> Batch, and just tell it to run that action on every file in the work directory.
Then it just zips through the images, producing nice neat lil thumbnails (usually there's a lil more to it than that, like running an unsharp mask, auto-levels or whatever, but this gives the general idea).
Actions can be as simple or elaborate as you need, but if you use a particular technique regularly, or even if you're only using it to develop a single site - but using it a million times during the development of that site, it can be a GREAT time saver.
Another example could be for photo processing. Not every image needs (or should) be processed the exact same way for optimum quality. You might have 20 different techniques for bringing out the best in a photo, and which you use depends upon the quality of the original image. Now, the first half a dozen techniques you try on an individual photo might not work exactly the way you planned. There's a couple of hours wasted playing with 'em though, heh. If you put your techniques into actions, you can simply load the photo, click a button, and within a few seconds (depending on what & how many different commands the action contains, and the speed of your processor), you'll get to see the results, and be able to pick out the best method in mere minutes. Again, if you're doing a site for a client that has a lot of photos to go into a gallery (especially if it's a site for a professional photographer, heh), quality is key for public presentation, and showing work examples.
If you end up taking 2 or 3 hours to determine the best method of improving a photograph's quality on each photograph, it's going to take days to get them all there into the gallery, or you're just going to give up and use one technique (whichever's manually fastest) for all the photographs, and some will not appear as well as they could. Going with actions automates a lot of your processes, and gets the job done quicker - freeing you up earlier to find another client willing to hand over lots of cash
