The $40m Series C round will apparently be used to help London-based HouseTrip grow market share, partly by expanding into new territories. The company’s competitors in this space include the aforementioned Airbnb, as well as more Europe-centric rivals such as Wimdu and 9flats.

“We don’t really look at our competitors,” HouseTrip CEO Arnaud Bertrand insisted to me. “We are pretty convinced about our own vision.”

That vision is pretty simple: make it as simple to book a rental property as it is to book a hotel room. As Bertrand notes, HouseTrip is “not very social” – unlike some of its competitors, the site eschews the setting-up of user profiles and the like.

Bertrand also notes the company’s relative focus on the high end. No guest rooms here; these are all proper rental properties. And, in a way, that makes up for the security boon that can come from forcing users to set up or link to social profiles.

“We have a pretty massive quality control team based in Lisbon,” Bertrand said. “They’re in charge of making sure that all new properties go through a pretty strict vetting process. We don’t go after primary residences because we think it’s simply too dangerous.”

HouseTrip also used the announcement to reveal some numbers – apart from the big ‘$60m’ number that now represents its total funding to date.

The site now has more than 130,000 properties listed in over 15,00 destinations, and it reckons it’s on track for more than three million nights’ bookings between its 2010 launch and the end of this year. Based on recent figures, and with local rival Wimdu having cut back its operations recently, that probably puts HouseTrip in second place behind Airbnb.

It may well be the case that the holiday rental market is not a winner-takes-all game, but that doesn’t stop it being fiercely competitive. With its growing collection of big-league backers, HouseTrip is certainly placing itself well for the next phase of the fight.

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Google and the Ghost of Silicon Valley Past
• Opportunities and risks in the share economy
• 12 tech leaders’ resolutions for 2012

Borrowing from and lending to your friends: it’s a very simple idea that could, ironically, end up making money.

Here’s how it works. The lender uploads a tagged photo of what it is they’re willing to lend. The borrower searches for what they want, either through a text field or by scanning the item’s barcode.

If one of the borrower’s friends has it available to lend, then that’s what happens. If not, the would-be borrower gets referred through to iTunes or Amazon to buy the item (generating a referral fee for Why Own It).

The app is the brainchild of Hamburg-based Philipp Gloeckler, whose sustainable goods marketplace AvocadoStore recently turned profitable, and developer Nils Hausig. Stephan Uhrenbacher of Qype and 9flats is also on board as a business partner.

“I had the idea on holiday in Cape Town in January,” Gloeckler told me. “I was bored and thought it would be great to see what my friend had.”

As it turned out, nobody was providing such a service. Why Own It got founded in April, and several months later the first iOS app is out for the German market – despite the fact that it’s all in English. “Germany is just a test market,” Gloeckler said.

The next step is to launch in the English-speaking world – well, the U.S. and U.K. – at September’s Disrupt conference in San Francisco. Then Gloeckler will move to New York to run the business from there.

How come? Well, according to Gloeckler, the German market is too small and full of clones.

“If I have an idea which has an international scope, why stay in Germany?” he asked – an interesting stance to take, given Berlin’s growing reputation as an internationalization hub, but then again Why Own It doesn’t hew to that kind of e-commerce model.

So what kind of stuff gets lent through Why Own It? Fashion accessories, sports equipment, CDs and, according to Gloeckler, a dog. But that’s OK, because it’s all between friends.

“You don’t want to give your car to a stranger but you wouldn’t mind giving it to a friend,” Gloeckler noted.

But isn’t there scope for awkward situations, where you like someone but wouldn’t trust them with your car or dog? “Of course, but you can always block people, or just say you lent it to someone else. You don’t have to accept every request.”

I’ll leave you with this video, which depicts Gloeckler interviewing a bemused-looking Mark Ronson (I have no idea why) about the app. It must be nice to have people lining up to lend you stuff:

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Will cloud computing push the BRIC market to the front?
• Will Standardizing the Cloud Cause Clarity or Confusion?
• Google and the Ghost of Silicon Valley Past

First off, the expansion confirms Stephan Uhrenbacher as one of the German startup scene’s key players. Not only is he the founder of 9flats, but he’s also the founder behind Yelp rival Qype, a popular European recommendation portal.

“North America, one of the world’s largest travel markets, is the most important market for 9flats outside of Europe,” Uhrenbacher said. “The integration of iStopOver will strongly contribute to our growth in this region and will both open a completely new geographic target audience for us and double our offerings at the same time.”

With services such as these, solid customer support is fairly essential. 9flats says it will open up positions in its new Toronto office and effectively double the size of its current support team.

iStopOver’s existing customers will also get a boost from the deal – they’ll no longer have to pay booking fees, due to 9flats’s company policies. Details of the transaction were not revealed.

Looking at the competition, market leader Airbnb has around 200,000 listings. Rocket Internet’s Wimdu has roughly 50,000 and HouseTrip somewhere north of 100,000. But beds aren’t the same as listings – obviously, some properties have more than one bed.

“We chose to estimate the benefit of the transaction conservatively, but we estimate to be at 100,000 apartments/homes within three months after completion of the transaction,” Uhrenbacher told me via email.

So where exactly does the deal leave 9flats, relative to its competitors?

Considering the company was only founded early last year, the acquisition puts it in a pretty strong position, albeit still slightly behind HouseTrip and way behind Airbnb. Indeed, it gives 9flats almost the same user base size as that of Airbnb’s European operations alone.

Wimdu, meanwhile, finds itself lagging. It potentially has access to plenty of funding – Rocket raises money at an astonishing rate, with key investor Kinnevik having put in more than half a billion dollars so far this year – but, unlike Airbnb and 9flats, Rocket’s outfit has not yet been on the acquisition path.

It may be that buying sprees are the best way to catch up at this stage of the game.

Related research and analysis from GigaOM Pro:
Subscriber content. Sign up for a free trial.

• Will cloud computing push the BRIC market to the front?
• Will Standardizing the Cloud Cause Clarity or Confusion?
• Google and the Ghost of Silicon Valley Past