How can I have multiple USB peripherals connected to my Raspberry Pi at once? For example, I want to have a keyboard and a mouse plugged in at the same time.
The Model B has two USB ports, so if that's what you're acquiring you won't have an issue (at the time of writing, 15/06/12, the B is the only model currently available.
If you're using a Model A, or you want additional devices as well as a keyboard and mouse, you'll need an external hub (that is, a USB hub with its own power supply.)
It connects to the Pi via one USB port and provides several others which all share the available bandwidth:
If you choose to use a hub make sure it's powered by an external power supply, (not bus-powered.) This will ensure all the devices connected receive the power that they need.
Searching for "powered USB hub" on most sites that sell electronic goods will bring up what you need. This was the first result with such a query on amazon (bear in mind that's the UK based site and comes with a UK power adapter.)
You can use a USB hub for that. From Wikipedia,
Make sure that the Raspberry Pi has enough power to power itself and the USB hub, or else the Raspberry Pi may become unstable or fail to boot. This can be ensured by using a reputable powered USB hub.