If you are out of the house and need to connect your MacBook to the Internet you can use your iPhone's personal hotspot function. This allows you to connect through your iPhone's mobile data connection. This is handy when you'd rather not use public Wi-Fi or none is available.
If you need to share a single image quickly and easily, without using iCloud, Dropbox or some other service you have set up, you can use Imgur.com. You can upload an image without signing up and then use that to share you images in emails and message boards where images may not normally be supported.
If you have ever wondered how data gets from a website to your Mac, you can use Traceroute to see the full path data takes through the Internet. You may be surprised by how many hops it takes to load a webpage. Viewing a few trace routes will help give you an appreciation for how complex the Internet we take for granted can be.
You can use various websites and apps to test your home broadband speed. Compare that to the speed you are supposed to be receiving from your provider and call them if you see a problem. You may be able to get better speeds without spending anything extra.
Every computer uses DNS servers to translate domain names to numerical addresses that can be used to find web pages. You can use the default DNS provided by your ISP, or you can choose your own. Sometimes you will see speed and quality improvements by using a public DNS instead of the default. Learn how to set your DNS in System Preferences and learn a potential pitfall of doing so.
If your carrier supports it, you can set up a personal hotspot with your iPhone so other devices like a MacBook or iPad can access the Internet. You can also connect with a dock cable or via Bluetooth.
If you have an Airport Extreme or a Time Capsule you can create a separate guest network. This appears as a different wi-fi network than your main one. Users of the guest network cannot see the computers or other devices on the main network. Setting one up in advance is a good idea so you can grant visitors access to your Internet connection without worry.