iPhone: Most websites these days use responsive design or mobile layouts to format content on your phone, but these sites aren't necessarily much lighter weight than the full page. If your priority is loading speed or minimal data usage, Instabrowser is what you're looking for.
Instabrowser claims to cut data usage by 90 per cent versus mobile Safari (depending on the site, obviously), which is great if you have a very weak mobile signal, or are just trying to avoid excess data. It accomplishes this feat by stripping away all of the site's formatting and CSS, leaving you with a single column of text (and images, if you want them). As you might expect, site formatting is a little hit or miss, and it might remind you of the pre-iPhone mobile web dark ages, but sometimes that's all you need. To help you navigate the newly-formatted sites, Instabrowser also attempts to parse it out into a table of contents on the right sidebar, letting you jump quickly between sections. If you're really lost, a quick tap at the bottom of the screen will load up the full site within the app.
I've tested Instabrowser on an elevator where I can never load a normal web page and it works flawlessly. It's not going to be my iPhone's primary browser by any means, but it's a great app to have handy when I have a weak signal, or when I'm competing for bandwidth with a load of other people at a concert or sporting event.
Instabrowser ($2.99) [App Store]