Update 10/22/17: This transfer bonus was supposed to end October 15 but has been extended through October 31, 2017.

Until October 15, 2017, you can get up to a 62.5% bonus when transferring your SPG points to American Airlines AAdvantage. Previous SPG to AAdvantage transfer bonuses have offered a maximum of a 20% bonus.

Transfers of American Express Membership Rewards to Virgin Atlantic miles will have a 30% bonus through November 30, 2017.

This is a fantastic deal that sets up a chance to book one way awards to Europe for 8,000 Membership Rewards+ $150.

You must transfer Membership Rewards in 1,000 point increments, so until November 30:

1,000 Membership Rewards = 1,300 Virgin Atlantic miles

Three Reasons Why You Want Virgin Atlantic Miles
1.

Tomorrow is the last day to transfer Membership Rewards to British Airways and Iberia Avios with a 40% transfer bonus. The cutoff is 11:59 p.m. MT on September 17.

At the beginning of July we saw the transfer rate between American Express Membership Rewards and British Airways and Iberia Avios improve from 250:200 to 250:250

In a surprising and awesome turn of events, right after the new transfer rate was announced, a transfer bonus was also announced.

Aeroplan's 30% bonus for transferring hotel points ends tomorrow night. If you have a stash of SPG or Marriott points, read this post to see if taking advantage of the bonus makes sense for your travel plans.

Through August 21 (11:59 ET), 2017, Aeroplan is offering the biggest bonus on record at the moment for transferring hotel points to Aeroplan miles. It's also not a tiered bonus like the others in the past have been. Transferring any amount will earn the advertised 35% bonus.

Citi is offering a 10% transfer bonus from Citi ThankYou Points to Eithad Guest through July 5, 2017.

If you have ThankYou Points earned by a Citi Prestige, Citi ThankYou Premier, or Citi Chairman card, then you can transfer ThankYou points to Citi's airline partners and are therefore eligible for the transfer bonus.
Using Etihad Miles for Etihad Flights
Etihad Guest Miles aren’t very good for booking First Class on the Etihad A380.

For instance, Etihad would charge 88,048 Etihad miles + $183 to book Abu Dhabi to London in First Class on the Etihad A380. 

During the transfer

Transfers of Membership Rewards to JetBlue TrueBlue points will have a 25% bonus through 11:59 pm MT, May 31, 2017.

This could be a good deal if you have an immediate use for TrueBlue points. In other words, if there’s a cheap JetBlue flight you want to book and you have ThankYou Points, pay attention.

Transfers of American Express Membership Rewards to Virgin Atlantic miles will have a 30% bonus through May 22, 2017.

This is a fantastic deal that sets up a chance to book one way awards to Europe for 8,000 Membership Rewards+ $150.

You must transfer Membership Rewards in 1,000 point increments, so until May 22:

1,000 Membership Rewards = 1,300 Virgin Atlantic miles

Three Reasons Why You Want Virgin Atlantic Miles
1.

Through April 30, 2017, transfers from hotel partners to Etihad Guest will earn a 20% bonus.

Your best transfer options are probably between Starwood Preferred Guest and Etihad, or purchasing a Marriott Hotel + Air Travel Package that contains Etihad miles.

Normally, 20,000 SPG points = 25,000 Etihad miles (thanks to the everyday 5k bonus you get on transferring in increments of 20k), so with the current transfer bonus, 20,000 SPG points = 30,000 Etihad miles.
As One Mile at a Time pointed out, normally for Marriott Hotel + Air Package 2, you'd pay 270,000

Transfers of Citi ThankYou Points to Virgin Atlantic miles will have a 25% bonus through tonight, 11:59 PM ET March 2, 2017.

This is a fantastic deal that sets up a chance to book one way awards to Europe for 8,000 ThankYou Points + $152.60.

You must transfer ThankYou Points in 1,000 point increments, so until March 2:

1,000 ThankYou Points = 1,250 Virgin Atlantic miles

Getting ThankYou Points

Citi Prestige Card: 50,000 bonus ThankYou Points after $4,000 in purchases made with your card in the first 3 months the account is open.