If you are interested in earning DL SkyMiles (which can be used on any Sky Team airline - including DL, CO and NW), you may be interested in the DL AX cards. I have had a personal DL AX card for many years, and just applied for a Business Card for my business.

The DL AX Business Card has a 15,000 mile sign-up bonus, plus an additional 1,000 mile bonus for each employee card you request at sign-up (note - there is no place that I could find on the online sign-up form to put names - I had to apply by phone), plus another 5,000 mile bonus when your spending reaches $15,000 plus another 5,000 mile bonus when your spending reaches $25,000!

The personal DL AX card also has the $15K/$25K bonus. Both also offer double miles for certain purchases (including food and gas) and a companion ticket each year.

I was told that even though I already have a DL AX card, I can still get these sign-up bonuses for the Business Card! ^ I only signed up a few weeks ago, so I can't say if I do (did) get the bonuses, but I did receive confirmation today that my card was approved. ^ I'll know in a couple of months!