Design x Metatooth is a web-based 3D modeling application. Check it out for yourself at design.metatooth.com! r8 adds Measure functionality to the basic View (Rotate + Zoom) capabilities. On screen help is non-existent, refer to the user documentation below.
User Documentation
Resize your browser width to 1000 pixels or more to access the menu bar.
View – Press ‘v’ to return to view mode. Zoom with the scroll wheel or middle button. Rotate with the primary (typically left) button.
Measure – Press ‘m’ to switch to measure mode. Click and drag with the primary button to get the 2D or point-to-point distance between two locations on screen.
Download – The download button in the upper right can be used to save the visible mesh to disk as a Binary STL file.