When IBM commissioned a typeface family for their own internal use, they also released it for general use. Clean and practical, Plex also has some style and warmth. With sans, serif and monospaced subfamilies and many weights, one might wish that many businesses relying on dull typefaces such as Arial and Times New Roman might make the switch and use something much better for free.
Another free offering, Overpass is not quite a grand as Plex, but with eight weights and true italics, it is a fine and generous offering. Very smart and highly readable, and more space efficient than Plex.
Hundreds of websites cater to the Internet's limitless appetite for free typefaces. Only a few restrict themselves to high quality fonts. Font Squirrel is one such, and Lost Type allows users to pay what they want for their font of choice. Some of the Font Squirrel offerings have multiple weights, and a few are issued by commercial type designers, such as Plex, a huge font family commissioned by IBM.