The current best one is:
Double EQM/EQS offer
Travel dates:
September 3 - December 15, 2009
Offer:
Earn your choice of either double Elite Qualifying Miles (EQM) or double Elite Qualifying Segments (EQS) on qualifying United® or United Express® flights.

Register for Double EQMs
Register for Double EQSs

You can only register for one. Even though you are just registering now it will go back and credit you for any flights flown in September, as per their conditions:



Also note:


Lastly, a brief explanation:
EQM = Elite Qualifying Miles. Typically these are miles that you have flown and do not include bonuses.
EQS = Elite Qualifying Segments. These are the amount of segments you have actually flown.

With this promotion you can choose to double either EQM or EQS. If you are flying longer distances it is better to do EQMs. If you are flying a lot of segments (connecting flights) then do EQS probably. This will help you upgrade to a higher status faster.

Australia bonus offer
Ticketing & Travel dates:
May 1 - October 31, 2009
Offer:
Register, then purchase and complete a paid, qualifying roundtrip between the U.S. or Canada and Australia and earn:

* 10,000 bonus miles for United First® (F,A,P)
* 7,500 bonus miles for United Business® (C,D,Z)
* 5,000 bonus miles for United Economy® (Y,B,M,H,Q)


Register Now

Fly bmi for double miles
Ticketing & Travel dates:
Now through December 15, 2009

Offer:
Offer: Fly on bmi between now and December 15, 2009, between London Heathrow and any of their destinations in Europe, Africa, the Middle East or Central Asia and earn double miles. The double miles offer is valid on any qualifying paid published fare in business (C, D, J) or economy (Y, U, L, S, O, B, K, M, H) booking classes. When you fly United® to London, you can:



* Get through check-in for all bmi flights;
* Get Get convenient connections in the same terminal at London Heathrow;
* Have a choice of lounges, either visit bmi Number One Heathrow or the Star Alliance® departure lounge;
* Shop in the terminal between flights.

United, United Express, bmi and ten other Star Alliance airlines now operate from London Heathrow’s Terminal 1. United moved into new facilities in Terminal 1 last year to be closer to the other Star Alliance partners. Customers now have a seamless travel experience with improved connection times and upgraded facilities.

In June of this year, bmi opened its flagship international lounge - bmi Number One Heathrow – which offers a quintessentially British environment in which to work, relax, dine and refresh. bmi Number One Heathrow and the Star Alliance lounge are open to first and business class customers, as well as Star Alliance Gold members traveling with bmi or any other Star Alliance carriers on a same-day international ticket.

You don't have to register for this one.