Here are some drills that I use in/connected with my swim sets...

1. Catch up drill. While gliding, have one arm out in front, and essentilly have the other hand  "catch-up" with a normal but held-for-much-longer stroke. Then, do the other arm and repeat back and forth.

2. Side/rotation drill. Start off on your side with 1 arm stretched forward as in a normal stroke. Kick about 6 beats on one side, then make yourself go through a full rotation to the other side, with the other arm stretching forward (as you would if in an elongated stroke). Kick 6 beats, and then continue going back and forth.

3. Fist drill. Go through the normal stroke - except with a closed fist. Concentrate on the rotation.

4. No legs drill.  You may want a pool side or lane lines to grab, in case you have problems with this. Put a thick band around both legs, such that they are tied tightly together (I've found an old swim goggles band tied together works well) . Push off and use only arms (you'll only be able to use arms anyway -- except for maybe a dolphin kick). You will be forced to perform your stroke with your upper body really forcing itself downward and streamlined into the water  -- otherwise your useless legs will start to hang lower and lower.

5. Hypoxic drills -- Perform a normal freestyle stroke, but make the breathing be on the 5th, 7th, 9th, etc stroke. Go as long as you can go between strokes.  If you are a one-side breather, it will be 4th, 6th, 8th, etc between strokes.

My warmup and drills usually looks like this:

warmup-

100 m -   easy swim

100 m -  swim with float on my legs

100 m - kick using the board in my arms.

drills --

Do the 5 named drills I listed above -- 50 m each (2 lengths each in a 25 m pool), with 5-10 sec recovery at the end of each length.

After my main swim set -- when I'm good and relatively tired.

I do 8 x 25 m kick sets, with about 10-15 seconds recovery. 

I think coyotecoach generally has it right. Drills -- while definitely capable of making ones stroke more efficient, stronger, etc, won't over the long haul be the thing that gets most of your speed up.

You can find a variety of drills to work on in various triathlon books.  I like those in Fitzgerald's mambo (i.e. major size) -- Essential Week-by-Week Training Guide.

Here's the list without adding any real detail:

chest press,  side kicking, layout freestyle, catchup freestyle, cheating catchup, count stroke, fingertip drag, fist, bilateral breathing, and sighting.

I myself generally do 4 drills during each swim -- catchup, side kicking, fist, and hypoxic(breathing on 5 th,7th ,9 th stroke)  --

each drill done 2 lengths (25 meters) with a short break between.

Following Fitzgerald's plans -- I usually do those 4 drills after my warmup, which is usually 100 swim, 100 pull buoy (i.e. arms only), 100 kick (legs only) -- and most of his plans have an added kick set just before cool-down.

If you want speed -- just as with running & cycling -- you have to do some forms of intervals/tempo workouts.

A sample idea of this  would be -- if you can say currently swim non-stop for  only 100 meters -- then  do an all out 25 meters/yards (i.e. 1 length in most pools),

then rest till nearly full breath is back, then do that again. You might want to start out with only 4-6 of those. Using the pool clocks, see how long it takes to get your breath back. If it's 40 seconds the first time, then cut your rest by 5-10 seconds the next day you do those (and don't be swimming on consecutive days). When your rest time gets down to 20-25 seconds, then start adding more intervals and/or increase to some 50 meter sprints.

Similarly, as your non-stop swim distance increases. You want to do what are tempo swims (not all outs like a sprint, but essentially sprint tri race pace).

If you can swim easy non-stop for 400 m, then good tempo swims would be  like  150,  200, 250, etc. You want to be breathing hard when you finish this, but you don't want to have to stop during them (i.e pace yourself to that distance, or else go down in distance). A sample would be like 2 to 4  x 200 m/yds with a 90 second rest As with the all-out sprints, over weeks and months you want to reduce the rest period on these down to say 30-40 seconds; and then add distance.

And a by-the-way .. you'll find that interval/tempo workouts done running and cycling will work together, such that speed gained in one will help make gains in the others.