CritterPet (formerly SpiderPet) allows for the ability to tame spiders and other creatures to be usable as pets - much like wolves - and most can be used as rideable mounts.
Some tamed mobs will defend themselves and their surroundings from hostile mobs, though they may not actively pursue the target. Many also apply some kind of effect to creatures they attack.
Taming
Simply craft the taming wand and load it with the appropriate items. Once the tool is ready, right-click the target animal with the tool. The tool will not be consumed, but the item within will be. Taming can be extremely dangerous due to proximity to a hostile mob, so the taming process grants some experience.
To heal a spider, feed it steak. Other animals may need different items.
Some animals can be made to sit; To make it sit, right-click it with a bone. Sitting animals will make less noise and will not wander.