Requires the library mod Collective.
This mod is part of The Vanilla Experience.
Spiders Produce Webs is a minimalistic configurable mod which makes spiders and cave spiders periodically produce a web when a player is close. Webs can only be produced if the spider is on an air block.
The effect in action with the delay set to 40 ticks (that's normally way too quick imo):
maxDistanceToSpiderBlocks (default = 32, min 1, max 256): The maximum distance in blocks the player can be away from a spider in order for the it to produce a web periodically.
spiderWebProduceDelayTicks (default = 12000, min 1, max 72000): The delay in between spiders producing a web in ticks (1 second = 20 ticks).
The feature was originally planned by Notch on October 23rd 2009, but never implemented: