I am going to buy the Pi Zero (when they have more in stock...) and with only one USB port I need a USB hub to connect my keyboard, mouse, and WiFi dongle. I probably won't be connecting anything else in the near future. Will I need a powered USB hub to use these 3 peripherals, or will an unpowered one work just fine?
Like @SteveRobillard stated in his comment, it's probably best just to buy the powered hub first and save you the possible trouble of not having it in the first place if you needed it.
There are OTG [On the Go] hubs, which are micro usb. They work a bit differently than a standard usb hub, which uses a barrel jack for power. This one from eBay is extremely cheap, and it doesn't look like it has a slot for power, but the port on the far left (in the below picture) is the power port. It takes the standard micro usb 5V 1A cable, just like the Pi, so it shouldn't be hard to find a power cable for this hub, or any other OTG Hubs.
I advise you don’t fret and choose usb hubs, which has many ports with different connectors. I have this one. Model hi-speed 7-Port Ultra-Mini Hub provides USB ports for Plug-and-Play connectivity to USB 1.1 and USB 2.0 devices. Data transfer rate of this hub reaches the level of 480Mbps.
I reliably power my pi and ANYTHING I connect to it with a Transcend usb 3.0 hub. The usb 3.0 part does not benefit pi (yet) however, EVERY port is powered. I went through a couple of hubs to find this one.
I have had mine since March. I checked, its still available.
You may not need a powered USB hub for your setup, but there can still be advantages to choosing one over an un-powered model. Some powered USB hubs can be used to power the raspberry pi as well by plugging the raspberry pi into one of the USB ports on the hub. I prefer this because it leaves just one power cable going from your setup to the wall, and makes sure that your peripherals have plenty of juice.
This page shows a listing of many powered USB hubs with notes showing if they can be used this way: http://elinux.org/RPi_Powered_USB_Hubs
Personally, I went with this one: http://www.amazon.com/gp/product/B005P2BY5I . It is tiny (same lenght as a Pi Zero, and just a little wider) and powers the raspberry pi with no problem. There's even a picture of this setup on the amazon page for this product.