Our 20mo can identify most of the letters, but in and out of order.
I've been working really hard with him on the alphabet - he doesn't know it's work, though.
Some tools we have used:
Lots of reading and pointing out the letters.
Singing the ABCs multiple times a day in many different ways, like a game
Fridge Phonics (by LeapFrog)
Alphaberry (by B)
We don't miss an opportunity to point out letters and reinforce what they are. Always have him echo you - it is very reinforcing, more so than just telling him what it is.
ETA: Perhaps I should clarify...
I'm not the learning police, nor do I push my son in any way. There is nothing wrong with exposure and immersion in our language at a time when toddlers are naturally making connections. I don't sit and drill him with flashcards, for heaven's sake. He's naturally curious and I take the opportunities as they naturally present themselves so it's very organic, not forced.