Initially only camera repairmen cleaned sensors. Their inspection devices ranged from a jewelers loupe with a detached light up to a stereo microscope.
The first device mass marketed to the consumer for inspecting their sensors (outside of a making a test image) was the 5X SensorScope by Delkin Introduced in 2006.
Soon after VisibleDust came out with their 5X version and then soon after they introduced their 7X version.
GGS introduced their first prototype DSLR Sensor Loupe at PMA 2008. They introduced a production version at Photokina 2008
LensPen introduced their SensorKlear Loupe at Photokina 2008
Eyelead introduced a prototype of their Sensor Loupe at Photokina 2008