Most of the players rush using a great deal of units in the middle stage of the game. Rushes are mainly consisted either of hydralisks or mutalisks. Zerglings usually tends to be abused (even though they are a very powerful swarming units). Also, spells are rarely used. Here are some of the build orders for making good early/mid game rushes and some advises for using spells.
This build order rally on the zerg build-up written in the zerg early game strat section, this is just a conclusion.
1. After you have started building your 4th creep colony, hatch another drone and build a hydralisk den and 6 more zerglings (3 eggs),
2. After the den has finished morphing, build 3 hydralisks and an overlord,
3. After that, keep pumping hydras until you got 12, then build an evolution chamber and expand,
4. If you happen to be rushed, keep your hydras behind the sunken colonies, and shoot, while sunkens are taking damage,
5. After you have destroyed the rush, expand and attack at the same time,
6. You will either severely cripple your opponent or completely kill him. Note that when you expand, build 4th hatchery as well.
A very useful upgrade for a hydralisk is their speed, with it they can easily evade psi-storm, get to tanks, guardians or reavers and dispatch them quickly. Don't forget to upgrade their range as well.
Even though you pay much more for an mutalisk than a hydra, mutalisks have distinct advantage of being flying units and can very easily do hit & run attacks on workers or no-anti-air units.
1. In the initial build order...
2. ... after you started building your 4th creep colony, hatch another drone,
3. Don't morph 3rd and 4th creep to sunken colonies as yet, first build a spire,
4. Morph the rest 2 sunkens as you get minerals,
5. Build 2 more overlords as you get enough resources,
6. Wait for spire to finish and build 6 mutalisks.
When the mutas finish you'll probably be rushed, what shouldn't pose any problem now. Keep cranking mutas until you have 12 of them, and then attack (note that you'll need additional overlords).
Queen has a powerful spells, but most of the people doesn't use them at all. I mentioned use of parasites earlier in gameplay section, but don't get carried on using it, because it takes far too much energy - 75. Second queen spell is ensnare is a very useful spell that works like stim-pack but opposite. Affected units won't just be slower moving, but they'll have slower fire rate what is good if casted over tight group of hydralisks, mutalisks, zealots, dragoons, marines,... Ensnare also reveals cloaked units what makes it even more useful. Third queen spell is spawn broodling, although sometimes useful it just takes too much energy to be used constantly (most of the time, two casts of ensnare will be much more useful than broodling). The best use is surely against ultralisks, then templars and siege tanks. The last, fourth spell is infest command centre which takes no energy. The terran command centre that you wish to infect must have it's hit points red. After that you'll get a new building, and a new unit to produce - infested terran (more about it later). Use this spell whenever you have chance, not because it's free, but because it gives you opportunity to build infested terrans which are no doubt one very useful ground unit.