What do social search manager Aardvark, collaboration toolmaker AppJet and email search appmaker reMail have in common? They are startups that were all recently acquired in multimillion-dollar deals by Google after having been founded by former Google employees. So now those founders are returning to Google to work on similar projects, with liberal re-signing bonuses in their pockets.

It’s not unusual for employees to leave and new fresh faces will be seen as better prospects. Google is busy launching enormous products like browsers and phones, consequently individuals with cool little ideas may well be better off exploring and testing them outside the company. It undoubtedly would have been easier for its HR department had Google managed to keep these people employed by offering them opportunities internally. At the same time, it’s hard to feel bad for a company with $24.5 billion cash on hand when it has to go out and spend a few million to recruit from the office park down the road. Former Google employees tend to be either wealthy enough to fund their own projects, or at the very least well-résuméd enough to secure backing.

It appears that Aardvark may be the only one of this trio of acquisitions whose product survives intact; the social search engine is now in Google Labs. Aardvark’s team had formerly worked on Google projects such as AdSense, News, Firefox and machine learning. Meanwhile, the AppJet folks (who’d previously worked on Google products such as Search and Health) are teaming up with Google Wave, and reMail founder Gabor Cselle (who formerly was a Gmail intern) will be a Gmail product manager.

Those startup products all well-designed and engineered and served a purpose, but none found Google-like scale on their own. Development without Google’s global scale may have, in fact, been a benefit; the blunders of the Google Buzz launch showed that a new web product from within the company would have been better suited to a more humble rollout.

If Google’s acquisitions continue, what might some of Google’s next targets be? Some other former Google employee web startups include Ooyala, TellApart, Red Beacon, MyLikes, OpTrip, Cuil, imo.im, Chai Labs and Howcast. Others have also taken a liking to Google offspring; for instance Facebook bought FriendFeed and Twitter bought Mixer Labs.

Google’s not the first tech company to try to invigorate itself with former workers; Cisco pioneered the concept of a “spin-in,” which often involved investments in former employees’ startups and options to buy them.