I like the Chase United Mileageplus Explorer Card because:
United is the best carrier for flying between my nearest airport & the airport I like to travel to. Cardholders not only collect miles, they get one free checked bag for everyone in their party and 2 free passes to the United airport lounge per year (comfy places to hang out with free booze, snacks, and wi-fi, in case of a long layover or flight delay).
I find that of all the frequent flyer accounts I have (Delta, US Airways, SW, American, and United), United is the easiest with which to get the free flights on the days & routes that I want. They have a very flexible redemption program: you can plan ahead & get flights for the lowest number of miles, but you can usually get last-minute flights too, though for more miles. It is easy to book one-way flights.
If you shop online, you can go through the United.com portal and get multiple miles for your purchases with occasional mileage bonuses (mileage varies but you can get, for instance: 8 miles/dollar at drugstore.com, or 3 miles/dollar at Apple; it can really add up).
You start with a 30,000 mile bonus (enough for a free flight between the US & Canada) and one no-fee year, so it's at least worth a try to see if it works for you. I just added a United business card, not only for the mileage signup bonus, but also to keep my business expenses organized separately, so I have 2 United mileage cards, and so does my husband--we have flown free between US & Canada three times this year, so it's working for us.
The chase.com website is easy to use too. Of course, don't get this card if you're not going to pay your bill in full and on time!