Carbonite, a Boston, Mass. company that offers an easy online backup service to consumers and small businesses, has raised $15 million more in a second round of financing.

The round was led by Menlo Ventures, and included 3i Group and Common Angels.

Carbonite appears to have some momentum because of the simplicity of its plan. People pay $50 a year for unlimited backup. That’s pretty competitive. It’s easy to use: You sign up on the website and Carbonite guides you through the rest of the steps. It has not disclosed how many customers it has, but the size of the investment suggests the company must have traction.

This is a competitive sector, with plenty of competitors. But many aren’t as focused as purely on backup. See Terence Pua’s review of the other players, who include Mozy, EVault, Sharpcast, Omnidrive among others

The company has now raised $21 million.

Carbonite said it will launch a Mac OS X version this summer.