External Tool, using Learning Tools Interoperability (LTI) is one of the simplest but most powerful tools that Moodle (and other LMSes) offer to teachers and learners. Edu-Apps.org is home to loads of great apps that have been all added in once place with easy to use instructions to setup access to the tools from any LTI compliant site.
Recently a handful of additional tools were added like Examity, Questionmark, Pearson MyLab, Rosetta Stone and Wiris Math editor. You can check out all of the tools, grab their configuration URL and see instructions on how to integrate. Check it out at https://www.edu-apps.org/index.html.
Note that not all are made to work with Moodle as is the case for WIRIS which has a Texteditor Plugin available for Moodle already.