I remember seeing a $5000 bill once when I was a kid.
Yup. We were once a free nation and the government didn't yet consider us its slaves, and thus restrict us from transporting large sums of cash. Back when that $5000 bill was printed, a brand new car from Ford would have probably cost around $500. That means that note had the actual value of about 10 new automobiles. Today, an equivalent bill would probably be a note worth several hundred thousand dollars.
The $5000, $10000, and $100000 notes were generally used as interbank notes, as opposed to actual circulating bills. $500 and even $1000 bills did circulate, though obviously the velocity of such denominations was somewhat low. If we actually lived in a Free Republic, you'd still be able to make use of $500 and $1000 bills, but our government doesn't believe its slaves/serfs should be able to carry that much value in a single bill anymore. I'd say $10,000 bills would be much more useful today for citizens, given what inflation has done to our currency.