It's been a ho-hum year for most investors. The S&P 500 is essentially flat for 2015 as of the Dec. 15 market close, and the Dow is trading slightly lower. This doesn't mean all stocks have been merely marching in place. There have been a few big companies -- many that you probably know -- that have rewarded their investors with huge gains this year.

Let's look at some of the stocks that have more than doubled year-to-date through Dec. 15. 

Netflix (NASDAQ:NFLX): Up 143%
Buying into the worldwide champ of premium streaming video hasn't been a house of cards for investors in 2015. Netflix's digital platform continues to explode in popularity. Its global subscriber base has grown from 53.1 million to 69.2 million over the past year.

Its combination of original content and shrewd licensing deals have made the service untouchable. Binge viewing is a thing. Netflix has revolutionized the way we consume television, and its shareholders are collecting the spoils. 

Wayfair (NYSE:W): Up 131%
Some IPOs need some time to get going. Wayfair went public at $29 in late 2014, but it was trading in the high teens by the end of last year. That gave it a compelling starting line for 2015 as customers took to its online storefront selling furniture. 

Pitching loveseats and daybeds online is no easy task. Fulfillment isn't cheap on heavy merchandise, and folks like to physically kick the tires of home decor. Wayfair is standing out with aggressive marketing, a broad catalog, and the value proposition of free shipping.  

Smith & Wesson (NASDAQ:AOBC): Up 126%
It may be odd to see a firearm maker on this list in an era where the cry for gun control is getting louder, but that's pretty much the point. Smith & Wesson sales tend to spike when the public thinks that the government's about to tighten gun ownership laws, and that's essentially what's happening now.

Net sales posted a 32% year-over-year pop in its latest quarter, and analysts see sales growing at a 15% clip for the entire fiscal year that ends in April. Naturally if gun ownership becomes more restrictive it will eventually weigh on Smith & Wesson's prospects, but for now the market's rallying around its latest results. 

Stamps.com (NASDAQ:STMP): Up 125%
Postage seems like a sleepy industry, but Stamps.com has managed to turn it into a hotbed of growth. Stamps.com posted year-over-year revenue growth of 37% in its latest quarter, and adjusted income is growing even faster.

Stamps.com initially stood out with its product where customers can personalize stamps with their own photos, but these days it's thriving with online postage and shipping software. Another thing that it's doing right is blowing through Wall Street's profit targets with ease. Stamps.com has beaten analyst quarterly earnings estimates by at least 16% over the past year, and that's usually a recipe for market-thumping returns.  

Amazon.com (NASDAQ:AMZN): Up 122%
The leading online retailer is only getting bigger, armed with tens of millions of Amazon Prime customers that are shelling out as much as $99 a year for unlimited two-day shipping on Amazon-warehoused goods. 

The market has forgiven Amazon's lack of chunky profitability. It gets what Amazon is doing as it builds a massive audience for its online shopping platform and digital ecosystem. Acceptance has been rewarding for its stakeholders, and Amazon is another market darling that has more than doubled in 2015. 

Rick Munarriz owns shares of Netflix. The Motley Fool owns shares of and recommends Amazon.com and Netflix. The Motley Fool recommends Stamps.com and Wayfair. Try any of our Foolish newsletter services free for 30 days. We Fools may not 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.