Overview of Units 1&2
Collocations are combinations of words which frequently occur together.
For example, 'solve a problem' and 'highly successful' are collocations. Browse the Collocations Dictionary.
Emoji in Macmillan Dictionary.
The Historical Thesaurus of English
The Phrase Finder
Online Etymology Dictionary includes interesting changes to known words and phrases, eg.:
Obviously for a long time online has meant "what I'm about to claim is not at all obvious, but I don't care to offer evidence or proof, so I'll pre-emptively bully you into feeling stupid if you think about it." Admittedly, it saves space. Interestingly seems to have acquired a similarly annoying sense.