I have one favorite TV show for teaching letters and it is the same one I watched as a child, Sesame Street. We love Sesame Street around here. A few years ago when Cole was a toddler, we did not have DVR so we didn't watch much Sesame Street. We were out and about when it aired and so it was a rare occasion to catch it. Now that we have DVR ( I love justifying my DVR with an educational purpose, but yes it still also tapes terrible things like Teen Mom) I record Sesame Street daily. I report to my four year old that we are watching it for his little sister, who as my husband would put it has the attention span of a gnat when it comes to watching a TV program but really often Cole gets the most out of it. Yesterday they even taught about simple machines like levers and wedges. I wish when Cole was younger we had this on instead of Diego. Their daily exposure to one number (1-20) and a letter comes through in a very integrated, modern, and fun way. I like how they use pop culture to make sure that they are up to date and yet still have some common threads present from 30 years ago when I was watching. Who wouldn't love to learn about the letter Y by listening to Norah Jones sing. Since Cecily still is working on that perfectly normal for her age gnat like attention span, we spend more time peeking at Youtube clips from Sesame Street that only last less than 2 minutes. A simple search with the words Sesame Street and the letter that you are reinforcing takes you to a multitude of clips. Our favorite this week is M is for Music. If you like Glee then you have to check out their G song.
If you are looking for a video that introduces letters and sounds then I recommend Leap Frog's Letter Factory. A friend recommended this to me last year and it really reinforced the letter sounds in a fun way. My sons still enjoys watching it even though he has now mastered this skill.
Again I only recommend using TV as an enhancer to other ways to teach, not the sole way to teach a skill. Keep it fun and functional. I know they say no TV under 2, but I believe everything is about balance and sometimes I need a break.