Swim Drills to become a faster swimmer?

swim, swim, swim and when you're almost done.... swim some more!

^^^This^^^, but generally speaking...

If you're already able to easily cruise at a pace of less than 2:00 / 100 for oly distance, I'd recommend a main set something like this after a good warm up:  20 x 100 on T1000 pace + 5 seconds.  This means if you can swim a 1,000 TT in 16:30 or so (about 1:40/100), your send off time for each 100 will be 1:45.  In this example, if you swim an interval in 1:35, then you get 10 seconds of rest.  If you swim it in 1:40, you get 5 seconds rest.  Strive to get a similar amount of rest on every interval.

If you're unable to cruise sub-2:00/100, then it's more likely that you would be better served spending more time on technique and less time on hard intervals.  It's impossible for anyone to say what your technique work should look like without seeing you swim.  Drills all serve specific purposes, and it's pointless to do them randomly without knowing what they're meant to accomplish.

^^^ what those guys said

and ditto on the drills ... all have specific purposes (or should), some of them accomplish those purposes better than others and are easier to integrate towards the whole efficient swim stroke. And that especially depends on what your swimming background is and what your stroke looks like to begin with.