What I mainly know David Tennant from, and what most people generally know him from as well, is Doctor Who, the fantastic Sci-Fi series on BBC. He played the Tenth Doctor and was on the show for four years before dying and regenerating into Eleven. Out of the three Doctors that I have seen (I haven't watched any of classic Doctor Who), I would say that Tennant is the one who gave the most to the show and set the bar pretty high for Matt Smith, the actor who now plays the Eleventh Doctor. Aside from Doctor Who, Tennant has been in numerous Shakespeare productions such as Love's Labour's Lost, Romeo and Juliet, The Comedy of Errors, and Hamlet where he was called "the greatest Hamlet of his generation." He is now starring with Catherine Tate in Much Ado About Nothing which plays through September of this year in London.I think what makes David Tennant such a great actor is how versatile he is. He can go from film to TV to the stage and to radio without skipping a beat. There are a lot of people out there who think that because they are awesome on TV or film that they will be awesome doing everything else, and that's not always the case. He is one of the few people whose talent stretches beyond one media because it can and not because it is forced to.
This is a link to David Tennant's official website. If you click around a bit you will find information on all of his projects like Much Ado About Nothing and The Gobetweenies, a radio show that he is currently taking part in. There are also interviews, photos and videos scattered about on related pages.
The link here is the IMDB site for Tennant. It gives you a complete list of everything he has been a part of in chronological order. Just by looking on the first page, I found out that he voiced one of the characters in How to Train Your Dragon, which is pretty cool.
This last link is a fan site for Tennant. I like fan sites like these because they tend to have content that other, more official site's don't. Big fans go out of their way to find pictures and videos that haven't been put up anywhere official quite yet.