I want to point out before I start that this is an 'average joe' kind of list; I know there are many more sites that I have yet to stumble across, and no doubt some of your personal favourites may be missing, in which case feel free to leave them in the comments.
It's not exhaustive, I just thought I'd share this list with you incase it saves some google-ing.
(also, IP1 have been super super amazing but they're more localised and I wanted to keep this fairly world wide)
In no particular order,
This site has a plethora of images, information and most importantly, ideas.
It showcases artists from around the globe and provides news dedicated to illustration. Anyone can join, but it's best to have your own domain name and some professional work under your belt to fit in with the sites very high, but very rewarding, standards.
It does a fantastic job of promoting its artists, and with the addition of a very comprehensive forum you will never be without answers to the barrage of questions freelancers are usually faced with (e.g. what to charge, contracts, copyright etc).
They also work closely with Escape from Illustration Island, which is by far the best illustration resource on the web.
This site screams professionalism; from its sleek interface and user profiles to the client lists various illustrators boast.
It lets you upload separate project galleries that fellow members and non-members can then 'click to appreciate', and with enough appreciation you can end up featured on the front page. Needless to say that in itself can provide a fair bit of promotion, although it is worth noting that a big percentage of the site's content is made up of advertising and product design which is done entirely digitally; so it may not be the ideal choice if your work is more personal and traditional.
Quite the opposite of the first two entries, deviantArt's contributors are as diverse as they come; from poetry to tattoos to artisan crafts there is an exhaustive list of the weird and wonderful ways in which people make art. I'm aware the site doesn't always get the credit it deserves, because of the varying quality of the art submitted, but that's what you get when tens of thousands of users are allowed to submit what they want (within reason). Despite that, as a community this site is second to none. The team responsible for running it provide numerous contests and rewards for the simplest of participation (badges, birthday offers etc.) and there are plenty of famous/established faces to be found, all of whom have found limitless promotion and commission work thanks to dA.
The site's target demographic is obviously teenagers, and this shouldn't be a bad thing, especially if you're marketing to teenagers. However, even if that isn't the case there is still every reason to create a profile here; you may not want to include it in your business cards or set up a prints account with them (poor artist profit margin), but deviantArt is well and truly for everyone.
On top of that, their portfolio service is a huge plus. If you don't have the money to get a portfolio site professionally made (and let's face it, some are extremely expensive if you're just starting out) this serves as a quality replacement. The portfolio layouts are simple and modern, they let your art do the talking and showcase each piece very well. The site also save you from being directly associated to deviantArt, with a choice of '.daportfolio.com' or setting up your own domain name in its place. There are things that still need improving (word limit for biography page), but overall it serves as a worthy stand-in while you generate the funds for a real site.
No list would be complete without twitter, although this wasn't the case a few years ago when the site was still in its infancy.
So, what is twitter? There seems to be a great divide in opinion among people when faced with this site; some see it as the ultimate indispensable tool to quickly connect with people and promote themselves, others see it as a waste of time because of the 140 character limit, and the rest have no idea what 'tweets' are in the context of social networking.
Personally I fall into the first two categories, because although I acknowledge how important twitter can be as a promotional tool (because everyone worth following is on the site) it's still best to use it in conjunction with your other more in-depth networking profiles, such as facebook, and update all of them at once. Relying solely on twitter could be counter-productive because even if you have a high number of followers, there will still be a percentage that won't have the time to search through their twitter stream to find your posts or click through to your profile. Twitter is meant to be ephemeral, that is my favourite thing about it, but this means it is not the most practical promotional tool. In the end though, you're always going to be better off embracing it, rather than letting the twitter epidemic pass you by.
This site has a little bit of everything. It has interviews, a forum, a blog, a shop (although it's closed for now) and the thing it's probably known best for - its weekly participation topics.
How it works in a nutshell: the site updates each Friday with a different word that is used as that week's topic, anyone is then feel to create illustrations in whatever medium they choose inspired by this specific word on their blog on website, and then link it back to IF in a thumbnail image. Some members are showcased on the front page for their efforts, and all seem to get more traffic to their blog or site from fellow members or curious lurkers. It's also nice to see people's different interpretations of the single word, and participating every week means that you never get the dreaded artists-block/have the opportunity to keep practicing regularly with possible rewards.