For U.S. stocks, 2010 brought the continuation of a remarkable comeback. The S&P 500 index has returned 15% year-to-date. It's up 86% from its March 2009 low--and just a 25% climb away from hitting all-time highs.

In a comeback year, it's fitting that two of the three biggest gainers among large, midsize and small companies, respectively, are companies that were recently left for dead. The third is one that's cashing in on consumer frugality evidence that households did not recover from the recent recession as quickly as share prices did.

Here's a look at these top gainers, selected from the S&P 500 index and its siblings, the MidCap 400 and SmallCap 600 indexes, and whether they're still affordable.

Large Caps

Cummins (CMI)
YTD gain (through Dec. 24): 142%

The S&P 500 is packed with cloud computing companies and other Wall Street darlings, so it might seem odd that a maker of heavy-duty engines has come out on top for the year. Between the third quarter of 2008 and the second quarter of 2009, Cummins shares plunged from $70 to $20. Now they fetch nearly $110. Cummins was quick to reduce costs during what looked like a synchronized global recession, and demand quickly returned, particularly in emerging markets, which now want all the mining trucks and power generators they can get their hands on. The company's sales are expected to top $13 billion this year for the first time since 2007, but whereas profits were $3.70 a share then, they're expected to more than double to $5.04 a share this year. The stock now sells for 22 times forecast 2010 earnings--a premium of about one-third to the broad market.

Midcaps

Netflix (NFLX)
YTD gain: 235%

I've been a Netflix naysayer, I'm afraid. I dig the service but have never considered the shares affordable, especially during a brief stretch in early 2009 when they gained about 20% while the broad market lost one-quarter of its value. At 65 times earnings, they're still too pricey for my tastes, but I have to admire the growth. Analysts expect sales to increase 30% this year and more next year -- and earnings to rise faster than sales. Killing Blockbuster has been good for growth. So, too, have deals that offer Netflix's streaming movie service through video game consoles, set-top boxes and handheld devices. (Asked by SmartMoney.com in July 2003 /investing/economy/a-conversation-with-reed-hastings-14540/ if the company would move into video-on-demand "once it becomes more widespread," chief executive Reed Hastings responded, "It's why we called the company Netflix and not DVD by Mail.") The recent recession has only helped the business, as customers swap pricey cable plans for $8 streaming subscriptions.

Small Caps

Crocs (CROX)
YTD gain: 208%

At $17, Crocs shares once again cost more than some Crocs shoes. Shares peaked at nearly $70 in October 2007 but sold for around $1 in late 2008 and early 2009. The company may owe its survival to its patented, rubbery "Crosslite" material, rather than a particular design (unlike, say, Heely's (HLYS), a maker of roller shoes), which has allowed it to expand from beachwear into shoes for winter, sports, casual-dress and jobs that require plenty of walking or standing (see: the Bistro Mario Batali Edition). Sales are growing by 20% a year, aided by new distribution deals with retailers in the U.S. and rapid expansion of the company's own stores abroad. Shares sell for 24 times this year's earnings consensus. At the risk of being Netflix-ed again, that seems pricey to me. However, profit margins at Crocs are only about half as large as those for Deckers Outdoor (DECK), a maker of furry boots and rugged sandals, suggesting Crocs still has plenty of room to improve.