Here is the timeline: USB 1.0 was released in January 1996. Windows 98 launched in May/June 1998. Windows 98 was the first MSFT OS with native USB support. The first iMac was launched in August 1998. USB 1.1 was released in September 1998.

According to a June 1998 CNET article titled "Peripherals to surge with Win 98," Rob Bennett, group product manager of Windows 98 stated: "There will be a pretty impressive showing of peripherals this summer..There are 250 devices due to be launched around Windows 98 and 100 in development [to be] released in the next year..."

(http://news.cnet.com/Peripherals-to-surge-with-Win-98/2100-1...)

Looking through Google news archives for 1998, I see the following devices with USB support: Logitech scanners, Intel video cameras ("Create and Share"), Imation and Iomega high capacity drives, Kodak digital cameras, joysticks, modems, etc.

Hence, there was a collection of PC peripherals supporting USB around the time of the Windows 98 launch (in May/June) and the iMac launch later in the year (August).