MIT Federal Credit Union is offering a certificate special with a yield of up to 5.80% APY for a 24-month term. This has a minimum deposit of $50,000, and it requires the Smart Money Rewards relationship which requires a checking account with direct deposit. Without this relationship, the yield is 5.55% APY, and with a smaller minimum of $500, the yield is 5.30% APY.
In addition to students and staff of MIT, membership is open to several other select employee groups. Unfortunately, I don't see any easy way to join if you're not in one of these groups even is you live in MA. Please refer to their Eligibility page for more details. Membership requires a minimum deposit of $5 into the savings account.
Branches are located in Cambridge and Lexington, MA. The credit union is federally insured by the NCUA (Charter # 3830). It has $220.8 million in assets and 23,381 members.