Right now, there aren't many "great" deals compared to the past.
All or most of the Airlines have scaled back to 30K bonuses, which after the 50K and 100K bonuses they were offering seems trivial. UA, SW or US Air are all offering at or around 30K miles. Citi has withdrawn it's AA's 50K bonus. Or, you could wait until after the AA/US Air merger to see if (1) Citi retains the program; (2) and, if they'll offer a new promo to launch a new card for the airline.
Non airline miles, there's the 40K bonus on the Arrival. Or, the 40k on the CSP, which is you best bet if you don't have it.
Sadly, I think we've probably seen the end of the great sign up bonuses.
+1 in that there aren't many good signup bonuses currently. Most of them seem to be fairly standard, and definitely nothing like the 75-100k bonuses we've seen over the past couple of years.
However I wouldn't say it's the end perse, but perhaps the end of huge public bonuses. I think we'll end up with more lucrative targeted offers, so it's not the end for everyone.
People have mentioned Chase United and Barclays Lufthansa. In addition, I know that the Amex Gold Delta gives you 45k miles and the Citi AA gives you 40k miles. The Chase British Airways offers 50k miles.
I currently have the US Air card (Barclay's).I signed up for it because upon sign-up I got 40,500 bonus miles (after you use it one time). You also get companion passes every year (Buy 1 ticket at full price and bring 1 or 2 people with you for $99). I also have Legacy Credit Card which gives me the same bonus on my purchases.