At the beginning of 2012, I set out to form what I oh-so-modestly called The World’s Greatest Growth Portfolio. So far, the portfolio has lived up to its moniker, returning 28% for the year and outperforming the S&P 500 by just under 10 percentage points.

The thesis for forming the portfolio was pretty simple: Invest first and foremost in companies that demonstrate exceptional levels of innovation, with special emphasis given to those that I believe will be around decades from now .

Today, I’m giving you the final set: Tier Two companies. These are companies that I consider to be highly innovative, yet I honestly have no idea if they will be around in 10 years. Each of the companies will get a 5% allocation in the portfolio.

I’ve reviewed 18 companies  for this portfolio, so without any further ado, here are the five that will be this year’s Tier Two stocks.

A pair of 3-D printers

If you’re not up to speed, check out a quick primer on what 3-D printing is, and what it could be. In essence, you’re looking at a small machine that could build just about anything, personalized, inside your home.

Last year, I included Stratasys ( NASDAQ: SSYS ) in my portfolio, and it has treated me well, returning 155% so far. Traditionally, Stratasys has focused on growing its business organically, primarily fueled by the record sales of the company’s Fortus printers for industrial customers.

In 2012, Stratasys also decided to go the acquisition route by merging with industry-peer Objet, a move that both the market and I think was a good move .

Unlike last year, however, I’m not going to be leaving out Stratasys’ main competition in this two-horse race — 3D Systems ( NYSE: DDD ). And I’m doing this even though the company has a history of growth by acquisition and is up an astounding 260% this year.

The bottom line is that both of these companies, while comparatively expensive, are sitting on a technology that could be as revolutionary and innovative tomorrow as the personal computer was yesterday. Of course, that may not end up being the case, but that’s why they’re Tier Two companies, with lower allocations.

Westport Innovations ( NASDAQ: WPRT )

Last year, Westport, a company that designs engines that can run on natural gas, was a Tier One company. In retrospect, that was a mistake on my part.

There’s no doubt that natural gas has the potential to be the fuel of choice for the future. We’re sitting on huge deposits of the stuff right here in America. And if the transition from petroleum to natural gas really does take place, there are tons of upside potential for Westport’s technology.

But there are drawbacks. primarily that a natural gas conversion is by no means a sure thing, and that Westport has to rely on joint ventures to manufacture its engines, as the company is only primarily a designer, not a builder, of such engines.

Yeah, it might seem crazy to invest in a social-media company that trades for 750 times earnings, but it seems like a risk worth taking to me.

LinkedIn has three different revenue streams and, unlike other social-media sites, advertising here makes up only 26% of all revenue. The other two streams come from individuals wishing to get their name out there, and companies paying to find the right people to fill their openings.

Over the past year, LinkedIn has grown revenue by 89%, and most of that growth has come from companies converting their HR departments over to LinkedIn. There’s a chance this is the face of the future for HR departments, and that’s why I’m investing in the company .

lululemon athletica ( NASDAQ: LULU )

I’ll be completely honest: I’m pretty sure Lululemon will be around in 10 years, meaning it should probably be a Tier One company. But there are two things that relegated Lulu to the bottom tier.

First of all, I simply don’t know how permanent the yoga/women’s-athletic craze will be. It could be permanent; it might not. I simply don’t know.

Second of all, the stock’s P/E, while lower than the highs reached in 2011, is still a pricey 45, and so I’m more comfortable making it a Tier Two investment.

Fool contributor Brian Stoffel owns shares of Stratasys, lululemon athletica, LinkedIn, and Westport Innovations. The Motley Fool owns shares of 3D Systems, LinkedIn, Stratasys, and Westport Innovations and has options on 3D Systems. Motley Fool newsletter services recommend 3D Systems, LinkedIn, lululemon athletica, Stratasys, and Westport Innovations. Try any of our Foolish newsletter services free for 30 days. We Fools don’t all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy .