eBird is the indispensable tool of today's birders. Binoculars, field guide, eBird. There are some "that's not the way I watch birds" holdouts, but by and far, over the past 15 years or so, bird watching has come to be synonymous with eBird.
eBird is a free real-time online checklist program with powerful tools that let you keep track of your own bird lists. Its power comes from its ability to also see the range, frequency, and abundance of birds around the world as entered by tens of thousands of eBird users. On a recent "Global Big Day" on May 9, 2015 there were 13,664 eBird users that submitted 42,920 checklists that contained 6,013 different bird species. One. Single. Day.
For entering your eBird lists in the field, there is nothing better than the BirdLog smart phone app by Birds In the Hand, LLC. It first hit the market in March 2012. Frankly, it was the reason I purchased my first smart phone.
The Cornell Lab of Ornithology, the host of eBird, recognizes the great utility of BirdLog, calling it "critical to eBird." Thus, the Lab obtained the rights to BirdLog and will make some changes and offer it FREE as eBird Mobile.
eBird Mobile is now in beta testing for iOS phones and will likely be available for Android phones soon. Since I have an Android phone I do not have a personal review for you of the beta version. In fact, I cannot find a review online... yet.
See older posts of mine about eBird and BirdLog on the now-idle "Pacific NW Birder" blog:
"What is eBird?" November 15, 2010
"BirdLog 'Killer App' for eBirders" March 19, 2012
"eBird Best Practices: Using Birdlog" July 26, 2013