A critical component of swimming strokes is the catch, the point in the pull at which you begin forward propulsion. A video from GoSwim demonstrates a drill for practicing a good catch in freestyle.
Box jumps are a great exercise to develop quickness and power off the blocks. Cullen Jones demonstrates them in a GoSwim video clip.
Have you ever used a Tempo Trainer from FINIS? Cokie Lepinski has published two tempo-based workouts on the USMS website: Basic Intro and Long Axis Focus.