Annotating the web isn’t a new thing. There have been a variety of tools that have allowed users to annotate text online and make it available to others. Diigo had an early version of this. There was a fantastic tool that 
Google killed off called Google Notebook that allowed you to grab snippets from the internet and post them to a notebook which could be shared (it was awesome, but like Google Reader, sometimes Google kills off awesome things). The Kindle and Kobo platforms allow you to share your annotations about books to others via social media. According to Delicious.com, I bookmarked a tool called Surfmarks back in 2010 that not only still exists, but looks pretty cool (although it’s a paid service)
Although all of these tools exist, there are two recent entries on the annotation stage that are worth looking at, particularly in a secondary school context: Genius.com and Hypothes.is.
Genius started out as Rap Genius, and specifically targeted lyrics, and the crowd-sourced analysis of the meaning of lyrics. They’ve branched out into other text forms now, but the site is still catering to a music loving crowd. Given the genre, the content isn’t necessarily appropriate for a school environment, but if you can find the right text/song, the engagement factor is incredible.
Hypothesis is perhaps the more logical choice, as it’s aimed at academics, and offers the opportunity to comment and highlight online texts. If you’d like to check it out, here’s a link to collaboratively annotate the new 21st Century Competencies document available on the Edugains site:
I’ve also added a new plugin on the Commons that allows you to offer readers the opportunity to annotate your blog posts. You can activate from the Plugins menu in your dashboard. The plugin adds the Hypothesis menu in the top right-hand corner of your site.
Shortcomings? You can’t highlight on mobile (you can annotate but the annotations are out of context). There seems to be a lot of momentum behind this tool though, so I expect this to change as they travel further along their roadmap. Educators looking to get started can check out their resources here.