I've linked a number of comics stories online to read here. Comixology is the only one I'm familiar with of the ones listed above. And I dislike that they want to charge you $2.00 per issue to read them online, for a comic that you wouldn't own. Relative to cover price, that's a pretty expensive "rental".
There are a lot of comics I can buy and physically hold and own forever for that price.
Especially for older stuff that sits a long time unsold in the back issue bins, I feel it should be available free, or close to free, to to promote work that would otherwise remain obscure and go unread.
I feel similarly about many movies and TV shows. Make them free and available to gain an audience they don't now have. If they become popular enough, then make them available on DVD or in series collections.