As a follower of several webcomics, I am truly amazed by this.
Ryan Estrada, a webcomic artist and
world traveler, not only convinced the creators of almost 50 webcomics to let him do guest strips for them, but he somehow also convinced
all of them to show his guest strips on the same day: September 17, 2007.
For whatever reasons, he is not showing on his own site a single list of all the webcomics he did guest strips for, but is instead putting up the actual strips, one at a time, at 5-hour intervals.
But other sites have been attempting to gather the list of all the guest strips he did. The best of these (still incomplete) I've seen so far
is here.
This must have taken Estrada a long time to prepare for. Consider it: not only doing that much art and trying to stay in the spirit of each of the comics, but also coordinating with that many sites to ensure they all put his guest strips up on the same day. Truly amazing.