For something a bit different, I would recommend
Do Androids Dream of Electric Sheep, but it is only available for online viewing, not for downloading.
It is a complete graphical conversion of the original book, including the entire text.
Oh, and even if you don't want superheroes, you might still want to check out Batman: Year One, and Batman: The Dark Knight Returns. I would have the latter up there with Watchmen for the best graphic novel ever.
For online webcomics,
Girl Genius (available as PDF downloads) and
Megatokyo, which oddly doesn't appear to by buyable in e-form.