Mendeley is freely available as:
Your Mendeley library can be synchronised across platforms, so you can access your Mendeley library from any device.
With the free version you have 2GB of personal storage space. It is possible to pay for a premium plan which gives you more storage space.
This Guide covers only Mendeley Desktop and Mendeley Web, which will continue to be available until further notice. We will develop support for the new Mendeley Reference Manager when it is fully adopted.
Mendeley have guidance on the new services:
A free Mendeley account gives you
To see how much storage you have left: