For fans of old words and synonyms, the Historical Thesaurus of English from the University of Glasgow may be of interest:
The HT is the only online resource to make every English word from the last 1,000 years and its meaning available to the public and fully searchable.
The HT contains a record of nearly 800,000 words used at any point over the last millennium. It also contains links to their synonyms and records when the word came into and disappeared from use.
Infodocket also notes that HT entries include direct links to the Oxford English Dictionary online, which Harvard subscribes to.