A lot of the girls will tweet when they're going on a cam. Some of them even have a link to their cam profile in their bio. So oddly enough, Twitter is actually a good place to find what you're looking for.

It depends on what you're looking for.  If you're looking to interact with a porn star, there aren't a lot out there.  Some of the porn stars have their own cam shows within their web sites.

The main one used by porn stars who don't run their own is streamate.com.  Avoid them.  Really.  The company is bad... and way overpriced.  They've killed my account twice now (with money lost) for absolutely no reason, and although I've been able to show them the "welcome" message they sent when I signed in that gives the username/password for the site, they insisted each time, that I never had an account.  Performance-wise they are ok but the customer support/service is truly awful. (cam4.com is the same company, btw).  All they want is your money and they don't care about anything else.

As for other cam sites: the most popular ones these days are chaturbate.com and freeones.com.  Both are free, and both have a few porn stars sprinkled in there and both have been at, and sponsored to some extent, the Exxxotica conventions.  Both are totally free unless you want group shows, private shows, or want to "tip".  But, you don't have to pay anything if you don't want.  

Both have a few porn stars on them - for example, just a little while ago I spotted Sophie Dee on freeones.com.  However, freeones.com is basically R rated - that is, while there's nudity and language, there is absolutely no sex.  They have rules that are supposed to prohibit the women from doing nudes in the public, free, cam sites but the rule isn't enforced very much.  But you won't find anyone on there having sex - the best you'll see is a woman sucking a dildo.

Chaturbate.com on the other hand, does have sex shows particularly in the couples areas.  I think you have to be registered to see them but it's still free again, unless you want a private show or want to tip.  And there are a few porn stars and starlets on there too.  

But, honestly, there are porn stars that do good shows and porn stars that do bad shows, and the same is true of the non porn stars.  On either site you'll find women you think are gorgeous (each man's taste is different) willing to bare all and perform on camera.

Btw, I suggest tipping, even if it's only a small amount.  The people on these sites earn money through the tips, and it encourages them to stay on the site.  If they don't make some money off their shows, they stop doing them.  They encourage the tips by doing things like not undressing or doing anything except talking until they have a certain amount in tips.  It's also considered polite to tip if you have a request.

Oh, and a word for certain folks out there (you know who you are): I do not make any money or get anything else in compensation for recommending any of these sites.  

I had streamate for about 2 years, i used to chat w/Lindsey Meadows, Kara Price, Smokie Flame, Heidi Hanson, and Sarah Vandella on there often. I decided to cancel my account & sure enough by doing so, I cant re-register w/them, which i think is stupid. Its ok but its not worth the money. I know theres flirtforfree (I know Avy Scott used to be on there often but i havent been on there in years), myfreecams, naked (dot) com (Mandy Armani is on there often), cammodels (dot) com (I think its a part of streamate) just to name a few.