Photos.com is now part of Getty images, the rest of this post is now out-of-date and for historical reference only - (pre 2009)
Photos.com is not a microstock site, I'd describe it as being at the lower end of the traditional full price stock image market (hence the images are of excellent professional quality when compared to some microstock offerings.)
I've written this post to mention Jupiterimages, their parent company, and also as I've mentioned photos.com on a number of occasions before. (Update: as of 20th February 2009 the stock agency side of Jupiter was purchased by getty images, the remainder of Jupiter media now trades as WebMediaBrands.) Jupiterimages run a whole network of photography and clipart sites online for many different market segments. They are an important player in the business and are responsible for the stock xpert microstock site and the photo.com Stock Subscription Service amongst many others.
Photos.com offers a subscription service for image users to download images, and have done this for many years, before the 'microstock bubble' started up. They source their images from their own collection of 7 million images and not from amateur or 'microstock professionals'; that said, as a photographer you should be aware of Jupiterimages and their growing collection of agencies and niche micro and macrostock sites.
Visit Photos.com if you want more info on their network of sites or if you are interested in their subscribing to their subscription image service.