I just bought one to help me capture images of various plant pathologies.
They are marketed as having a range of magnification from, for example, 20 to 300X and the cameras have resolutions anywhere from 0.3 to 5 M pixels. Be aware that the specified magnification range may include the magnification inherent in displaying the image on a large monitor !
Dino-Lite make a range of professional USB microscopes of various sorts but these can cost hundreds of dollars. For basic experimentation there are a load of other vendors and products with prices in the $50 - $100 range.
If you look on Amazon it will be obvious that the same basic models are being sold be several different vendors.
Here is the one that I bought for $60 which is branded DBPOWER
The same microscope with the same stand is also branded as :
Celestron 5 MP Handheld Digital Microscope Pro $83
And with a different stand, or without a stand, by a wide variety of other vendors
I went with the $60 DBPOWER variant with a 5 MP camera. It comes with the stand in two pieces and a CD containing software called MicroCapturePro for PC and Mac.
You need software to interact with it - it does not just show up as a camera in the MacOS ImageCapture or Preview tools.
I don't have a CD drive on the machine I want to use this with and transferring the software from one that does is a bit of a pain. So I looked online for the software.
You can get MicroCapturePro from Celestron but after digging around a bit I would recommend a different solution.
Plugable have a similar microscope and they provide a piece of software called Digital Viewer - get that and install it on your Mac (they have a Windows version too). When I started it up it displayed the view from my webcam but click the Settings icon in the top left and select the microscope.
The focussing mechanism on these microscopes is a bit rough but you'll get used to it. I can see that I may want to build myself a better stand and illumination rig.
Under Settings you can select the image resolution and do a bit of image adjustment. You can take individual images, videos or a set of timed images.
It looks like it will do exactly what I need it to do - for $60 that's not bad.
Here is an example image: