Behance - promotes itself as "...The leading online platform to showcase & discover creative work." They are also part of Adobe so have considerable resources.
Their portfolio system is clean, elegant, simple to use and functional on numerous platforms including mobiles. They also cover a broad range of creative activities. Even if you don't use them for your portfolio, take a look at some of the content people have posted there. There is some exceptional material.
Once you have set up your online portfolio, one very nice option is that you are given a web link which 'strips' out the rest of the site and produces something like a mini website for you. If you e-mail this to galleries, clients or curators, they just see YOU, not all the rest of the content from the site. A clever and effective way of putting you first. We don't know of any other free portfolio site which does this.
They have a paid for 'Pro' service which extra facilities and have received over 50 million project views in the last 30 days - so there is a lot of traffic to the site.