Google appears to be in a rush to get its holiday shopping done, having just acquired its fourth company in less than a month.

On Friday, AppJet, a start-up co-founded by an ex-Google employee, revealed that it had been bought by Google.

In October, Google CEO Eric Schmidt said his company would be looking more actively for acquisition targets. In 2007, Google acquired 16 companies. In 2008, it bought only two.

AppJet makes an online collaborative Word processing program called EtherPad that allows real-time collaborative authoring similar to what Google offers in Wave.

Fittingly, the company said that its EtherPad engineers would be joining the Google Wave team.

Google confirmed this: "Appjet, the company behind EtherPad, is a team of highly-talented entrepreneurs with deep expertise in real-time Web collaboration," said a Google spokesperson via e-mail. "Google and Appjet have a shared vision of how Web collaboration can benefit users, and we're excited to have the Appjet team contribute to the success of Google Wave."

While Wave and EtherPad appear to be alike in some respects, at least one EtherPad user claims that EtherPad handles collaborative editing better than Wave does.

AppJet says that the EtherPad site will remain functional through March 2010, though no new free public pads -- the company's term for EtherPad documents -- may be created.

After March 31, 2010, all pads and personal data will be deleted, the company says. Data in existing pads can be exported in various formats using the "Import/Export" tab.

Users of the Professional Edition will not be charged after today and those who downloaded the Private Network Edition can continue to use the software. The company will honor service and support contracts, though no new contracts will be offered.

EtherPad users who have supplied Appjet with e-mail addresses will receive Google Wave invitations before the end of the year, the company says.

Google's other acquisitions this year include On2, reCAPTCHA, AdMob, Gizmo5, and Teracent.