Grammar question: which vs. that

Bitt and/or other grammatically inclined folk, I'm having my latest grammar crisis.

What's the ruling on using "which" vs. "that" in situations such as:

"I found more features [which/that] I was interested in."

For some reason, I cross paths with this problem 10 times a day all of the sudden. Thanks.

We actually did which vs. that recently. Lemme find the thread.

Here's a quiz to occupy you while you wait:
http://webster.commnet.edu/grammar/quizzes/which_quiz.htm

Is it me or is question 2 missing on that quiz? Interesting.

Either will work in the example you provided, but "that" is probably preferred.

Here's a confusing explanation.

Here you go. My previous explanation.

Huh. That explanation's actually wrong.

Edit: Well, no. It's not wrong. Just far from complete.

I always use "which" for nonrestrictive clauses. For restrictive clauses, I use whichever word is the clearest, with a heavy bias to "that."

Thanks, I'll take a week or two to digest that.

Or, rather, "I'll take a week or two to digest which."