Magazine Man posted an entertaining story about being lunchmeat for a spider. Various commenters have told of their own experiences with spider bites.
Unfortunately, I've had some experience as well. I never saw the attacker. It was a drive by biting. Two punctures close together on my neck. Unlike MM, I had no physical symptoms. What was both interesting and gross though, was how the skin necrotized from the poison. The wound ended up being more than 1/4 deep and I am left with this scar:
Here's a closer look:
This happened 3 years ago. My advice to you is: Be wary of spiders, especially the ones you don't see.