I'm just starting out with PHP. As I learn, I get more ideas as to what I could use it for, but I'm still not seeing much.
So, what kind of cool/useful stuff can it be used for?

in addition to that, what tutorials/sites do you recommend for a beginner with this kind of thing?

Apart from doing my laundry (even my underwear :D) PHP handles logic. You can use it to set conditions on which to perform certain functions or actions. It's great at handling dynamic content.

You can create Content Management Systems (CMS), blogs, guestbooks...the list is actually very long :D But if you try and create something like a simple guestbook or a blog, you will realize how much it can do even though that will only be a fraction of what it can do. It seems like PHP has endless possibilities :D

Do you have anything particular in mind that you want to know if PHP can handle?

Even though I'm fairly new to it, I've actually just started making a small-scale CMS for a site I'm working on, I started on a blog aswell since I didn't like the way Movable Type handled skins, then I got lazy and discovered Wordpress haha.

For picking up tutorials I recommend Pixel2Life.com. It has loads of links to all kinds of sites with tutorials. Not just PHP, stuff from MS Paint to C++.
Of course theirs php.net, phpfreaks.com and devshed.com.

Personally, I'm not too keen on Devshed, but a lot of people seem to love it.

PHP is most useful when used in combination with a database, like mySQL, but I use PHP on most of my sites, sometimes just to include headers and menus so I don't have to repeat code.

As David has already said, www.phpFreaks.com is a great place, they have forums where you can post your questions/problems and tutorials.

Rob

The best part about PHP is how easy it is to perform functions that are very complex with other languages. Since PHP is dedicated to the Web first, it has many useful built in fuinctions. Over the past year or so I've been developing a CMS with PHP, and every new feature I add makes me amazed at how many useful functions there are. Then when you look at some of the stuff you can do by using PEAR with it.

If you want to see what can be done, take a look at the php foundry:
http://php.foundries.sourceforge.net/

You can learn by working with these programs.