Foreign Language Tools - Please refer to the main post.
Curation Tools - Full writeup here. (Note that some links have already been mentioned below with fuller explanations)
Formative Assessment Tools - Full writeup here.
Top Hat Monocle
LMS/SIS/LCMS/Social Networking - Full writeup here. Last 4 most promising for daily teacher/student use.
My Big Campus
Twitter Resources - Full writeup here.
Resource aggregators - Full writeup here.
Ted-Ed - Flippable videos on a variety of topics. Write up here.
Edjudo - Educational technology resource site. Write-up here.
Backchannels - Full writeup here.
Best (specifically made for backchanneling)
Good (not made for backchanneling, but can suffice or offer similar functionality)
MOOCs - Full writeup here.
ThumbScribes - Linear collaboration for writing narratives and poetry. Write up here.
Grade Cam - Alternative to Scantron sheets for fast grading. Write up here.