If you’re going to fill a cave, dungeon, or forest with Giant Spiders, there has to be a way for them to catch their prey, doesn’t there. To go along with my custom 2nd level arachnid I created a Giant Spiderweb trap designed to snag unsuspecting adventurers. Because it does no damage and is essentially one-shot (I reasoned that anyone escaping essentially destroys the web in the process), I decided it was a Minion trap and worth 25XP.
In play, the trap works well stretched across entryways or narrow spaces. And of course, in the process of trapping the adventurer, the trap is also likely to inform its creator (the aforementioned Giant Spider) that dinner has arrived.
Update: The Monster Manual 3 describes a terrain called web sheets (MM3 p.183) which is very similar to my Giant Spiderweb Trap design. It makes an attack versus reflex (which the Giant Spiderweb Trap should probably do to be consistent with trap design) but only immobilizes victims (can’t move). I think restrained (can’t move, -2 to attacks, grants combat advantage) is more consistent with how a giant web would work (and be more useful to the spider spinning it) but to each their own.