Putting Your Portfolio Online
Sites at Which Artists May Submit their Websites
On the list below, the arrow 
indicates those sites which generally send "hits" to my website
(after I registered my site with them). I haven't yet paid for much in terms of "fee required" listings and haven't yet listed my art at eBay, Indexed Visuals, or iTheo, so a lack of an arrow on those sites is not meant to indicate a lack of potential for sending traffic to your site, should you choose to list your site there. Two arrows 
indicates the site that currently sends the highest number of referrals to my website. Sites that do not regularly send traffic to mine are still worthwhile because search engines can see your site being linked to by other sites. This builds up the legitimacy of your website.
Google (this is also how you submit your site for Google Images)
There is a new option at Google, Google Webmaster Tools. When you register for an account and upload your site to be reviewed here, there is an option to participate in "enhanced image search". It seems to be a good idea for anyone having unique images on their website.
Open Directory's Category for Medical and Scientific Illustration (I am a DMOZ editor. See my DMOZ profile & bookmarks)
Interested in listing your site and/or your artwork at sites that require a membership fee? See
Listings Requiring a Membership Fee
For more listings, see Yahoo's List of Virtual Galleries
Also note that many of the artists' associations will link to your website from theirs if you are a member of their organization. See a list of artist's organizations here.
Finally, don't forget about your college. Often they will link to alumni web sites. It's worth checking into.