Technically, the first webcomic I read was Digger, but the one that actually got me into webcomics (by providing links to others) was Dresden Codak. Which appears to have updated several times since I abandoned it years ago.
Lackadaisy was also one of my favorites for a while, but it also still seems to be updating at a snail's pace. Which, considering the quality of the artwork, isn't altogether surprising.
Another early comic I really loved, and which I actually followed to its conclusion, is Archipelago. The creator is working on a new comic now.
I think there were lots of others, but those are the early ones I actually still remember.