A USB fan is a small and portable device that's handy for personal, close-proximity cooling. As the name suggests, it's powered through a USB connection, such as the one in your computer, a power adapter that you plug into a wall outlet or a portable battery. Some can be self-powered using an internal battery that you recharge — over USB, of course — and, in turn, some of these models can even be used as a portable battery to add some charge to a phone.
Despite their size, USB portable mini fans can be powerful, and most offer different speed settings. They won't cool down an entire room, but they can keep you comfortable when used at a close distance, such as an office desk or bedside table. Best of all, they are inexpensive.