For years, you could tell the hottest open-source projects by the number of programmers on an e-mail list, or by the number of books published. Increasingly, though, the hottest projects are those around whom service-oriented businesses are being formed. Yesterday, Ruby on Rails hosting company Engine Yard received a $15 million series B round from NEA, Amazon, and Benchmark. This follows an earlier $3.5 million round in January of this year.