Ais mentioned the "virtual box"...lemme explain:

I'm on a mailing list of mainly artists. (Where I actually learned of
letterboxing in the first place in April...) As Ais and I have become
giant devotees of this activity, and somewhat vocal devotees at that,
I've been getting -tons- of questions about what's in a letterbox, how
do you do it, and so forth.

Rather than continuing to explain, I popped up a little web-based
letterbox the other day, just to kind of virtually whet the whistles, so
to speak.

Clues are here:
http://www.livejournal.com/talkread.bml?journal=moderngypsy&itemid=137881

(The stamp, incidentally, is the new hitch-hiker. So if you'd rather
that be a surprise, don't look! :>)

Oddly enough, people have been going -nuts- for this now, and have been
wanting to make their own. So if you see anything about virtual
letterboxing, it's my fault. Sorry. :)

---e