This is an interesting list of technologies we used to have, but have lost.
Some of these you may have heard of (Greek fire, for one), some aren't really "technologies" (the library at Alexandria), but there's some interesting stuff here.
One thing I find amazing is Roman cement. Not only did cement technology get lost, we never really recovered it. Roman cement is much more durable than modern Portland cement -- that's partly why the Coliseum is still standing but we can't get a concrete road to last more than 30 years.