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.