A list of free schematic diagram drawing tools, such as SchemeIt, Draw and Falstad, is available at the Into Robotics website. Each of these opens within an Internet browser to be used online.
SchemeIt, available at the DigiKey website, is highly compatible with most browsers and is among the most complete tools of its kind. It can be trialed without registering for an account.
Draw is likewise replete with features and allows for easy export to a range of destinations, including Google Drive, Dropbox, OneDrive and the user's own hard drive.
Falstad is of a somewhat simpler, cleaner design powered by Java.
Other tools include CircuitLab, Webtronics and EasyEDA.