One of these is the the Highlight Tool, a very useful Google Docs add-on for students and teachers. Users can set up libraries of highlighter colors (pink for thesis, yellow for supporting evidence, green for subjective opinion, etc.) and maintain different libraries for different assignments or classes. Users can also copy just their highlighted text into a new document, either by color or by how the text occurs. This can be used for extracting essential information from a reading passage, or evaluating one's own written work.
Teachers can also set up a set of standard highlighter colors and export it to their entire class.
Here's how it looks in action:
(To install a Google Doc add-on, go to the Add-ons menu in any Google Doc, click on Get add-ons and then search for the title or feature you want. Once the add-on is installed, you can access it every time from that same Add-on menu.)
For annotating different documents for research, like websites or PDFs, there are also a number of extensions and tools to try. One easy-to-use option is the Chrome extension Weava. Weava lets you select text on any website and apply a variety of highlight colors, as well as take notes directly on the page for organizing and exporting later. Here's what it looks like:
(To install a Chrome extension, go to the Chrome Web Store and search for the title or feature you want. Then access that tool from the right of your Chrome URL address bar.)
Both tools are worth exploring!