does that mean you couldn't even bring a lighter or matches or eyeglasses or a pen? (isn't everything invented since that time advanced technology?)

You can bring those things, but once the lighter runs out of juice or the matches are all gone... you'll probably be tried as a witch and burned at the stake. You need something that won't run out.

A comprehensive encyclopedia.

i'd bring school supplies so that more people could read and write, and rebel and not be serfs anymore. And i'd make sure all the stuff they learned from were things like our Declaration or the Rights of Man documents and social justice pamphlets...

Knowledge is power... that's quite admirable, but if you REALLY wanted to impress the pants off those people? And not in a sexual way.

Where does 'advanced technology' cut off? Does ortho's encyclopedia require Quark or PageMaker or whatever, or just a printing press? Are guns verboten because of the advanced metallurgy or because of the gunpowder?

If we limit ourselves to medieval technology, the most impressive option would probably be, like, a big pile of gold and jewels or something.

Figure out how to make penicillin from what's available around me (plants, bread, etc.).

Have you read or watched Timeline or read Doomsday Book? Anyone have any suggestions for similar books?

I'd bring a time machine so I could get the fuck out of medieval Europe and back to the 21st century. Who wants to be surrounded by a bunch of plague-ridden, rotten-toothed, non-bathing peasants (other than Southerners)?

The Black Panthers.

Since they had "guns" by the early 15th century, I argue that I could just bring a more reliable gun and the formula for gunpowder. And maybe the Merck Manual of Diagnosis and Therapy.

A recumbent bike.

...but if you REALLY wanted to impress the pants off those people?

A polaroid camera and tons and tons of film.

Guns would be tricky, b/c you'd quickly run out of bullets and/or gunpowder. I guess you could bring a musket and find a blacksmith to build extra bullets, but making blackpowder would be tricky.

A solar powered calculator or slide rule, squeeze-powered flashlight, binoculars/telescope or something of that nature would be extremely useful, otherwise something like Henley's Formulas or A Barefoot Doctor's Manual (a microfiche viewer & several hundred textbooks would be extraordinarily handy).

The polaroid was my first thought too! But dependent on batteries and film ....

A book that predicted lunar and solar eclipses.
A high quality microscope and/or telescope.
A huge roll of kevlar
A wind up gramophone and the complete works of The King plus a few wailing banshees
A huge box of IUDs
Silly putty
The Book of Mormon
A few reams of blotter acid

Roller Skates.

Angelina Jolie.

My very own castle

All kneel before the Overlord of Fun! MUAHAHHAA!!

silly putty.

Keith Richards.

I'd lean to make a battery out of copper, zink and acids. then I'd burry under the future home of the cambridge archeology lecture hall.

The Way Things Work. Probably more valuable than an encyclopedia, which would have a lot of useless information about the "future".

This is often cited as an essential work in case civilization collapses (or is pushed).

And probably one of the most useful and exponentially influential inventions ever was the accurate self-contained timepiece, as detailed in Longitude. Medieval Europe might have been a tad too early for that development, but if I could travel east or south to the then-ascendant Arab civilization, I'm sure their navigators would have been happy to make use of it.

If the goal here is simply a short-term visit where you "impress" people a la explorers among the savages, I dunno -- that sounds like a weapon (the musket being the classic choice here) or perhaps explosives. Excepting the probability that you might be turned into local church authorities for sorcery, I'd choose something a little more useful with long-term impact. A hot-air balloon, for example, although the impact on castle warfare might be negligible. The astronomy calendar would be a good one for impressing the intelligentsia, but this being prior to the mercantile era, there probably wouldn't be much of a way (e.g. gambling) to capitalize on it.