The best thing I've had fun playing with recently is chaining in PW, and it's worth knowing about. Here's a good, slightly abstract example:
On a large communal news website, an Article page has a PageField called Author - this links to the Author's page. The Author works for a Company - there might be several authors in that company writing on this website and you might be curious and want to find out various things like the following (pretend we're on the article page still):
// Let's find all articles by this author:
$author_articles = $pages->find("template=article,author=$page->author");
// Let's get the company name that this author works for - going through a few pages to get this one - from the article to the the author' page then the company page - all using the incredibly powerful PageField field type <img src='http://processwire.com/talk/public/style_emoticons/<#EMO_DIR#>/smile.png' class='bbc_emoticon' alt=':)' />
echo $page->author->company->title; // Ridiculously simple huh?
// Now let's grab a list of other authors from this company - not sure why you would, but you could (the authors are all PW users in this case, so the template is 'users')
$company_authors = $pages->find("template=user,company=$page->author->company");
// And finally let's get a list of all articles written by all authors in this company using $company_authors above
$company_articles = $pages->find("template=article,author=$company_authors");
Now some people might wonder how the last one works - surely $company_authors is an object or array right? Well if you run a simple echo on it, it doesn't spit it's dummy out like PHP normally would, but rather gives you a concatenated list of page IDs in the format: 123|124|1024|456 - the pipe character is used in the last $page->find to mean "OR" so it works perfectly.
is why I keep getting excited whenever I work with ProcessWire. I would need literally dozens
of lines of code to recreate that example in any other CMS I can think of.
I pulled in the articles and authors by template, but there are various ways of doing it and I just did it with template names as that didn't assume any particular site structure, so it's not necessary to follow that exactly.