Domain parking firms like Sedo. Also profitable as ROI might mean a small unheard of startup could be more profitable than Google in terms of relative profitably but just on a smaller scale. For example, ecommerce B2C firms, selling moderately priced items, whose operation is only made possible with the web could theoretically be more profitable than the ones you mentioned.

Eg: "SnapNames and its competitors would then auction off deleting domains, generating an average of $100 per domain (with some going for $100k or more). SnapNames was recently acquired by Oversee.net for $35 million, based on yearly revenues of around $40 million and profits of $3 million or so."

http://www.techcrunch.com/2007/10/05/domain-industry-intrigu...

There are a huge amount of online businesses that are very profitable. This shouldn't take much research to figure out.

At Hotjobs we were doing significant revenues ($>100M) in 2000, and that was almost a decade ago.

On a more interesting note, start looking at net income for these companies.

Zoho -- $1 million profit/ month

What about Salesforce.com?

eBay, IAC, AOL/TW, RealNetworks, SalesForce.com, NetFlix, eTrade