Specialty-sector ETFs, or “thematic” ETFs, have become a hit with investors, covering more than 40 unique themes in the market with more than $10 billion in assets under management, according to IndexUniverse.

While some consider thematic ETFs “gimmicky” and believe these forms of ETFs are only popular when the media is hyping the sector, they have their advantages and an investor should be aware that such options are available.

Specialty ETFs can help round out a portfolio and allow investors to take advantage of different segments of the market. To spot opportunities, watch the trend lines.

Alternative energy. Skyrocketing energy prices have brought more investor interest to all sorts of energy investments. The largest alternative energy ETF is PowerShares WilderHill Clean Energy Portfolio (NYSE Arca: PBW), currently up 23.4% year-to-date, with $743 million in assets under management. PBW is more focused on U.S.-listed companies.

Coal. The cheapest energy source of BTUs. Ever-increasing oil prices are encouraging the development of the coal. The largest coal ETF is Market Vectors Coal ETF (NYSE Arca: KOL), currently up 1.3.4% with $277 million in assets. KOL primarily includes mid-cap miners, weighted 49% in U.S. companies, 23% in China and 15% in Indonesia.

Nuclear. Once operational, nuclear power is seen as the cheapest kind of energy, but the industry is still grappling with the issue of nuclear waste. The largest nuclear ETF is Market Vectors Nuclear Energy ETF (NYSE Arca: NLR),  currently up 21.7% year-to-date, with $166 million in assets. NLR has a 40% allocation to uranium miners, with other weightings in power generation and plant construction firms.

Commodities. ETFs are an easy way for a commodity trader to access this area of the market. The largest hard asset commodity ETF is Market Vectors Agribusiness ETF (NYSEArca: MOO), currently up 40% year-to-date, with $1.5 billion in assets. Other areas include water resources, steel, timber and broad-based commodities.

Infrastructure. Infrastructure covers companies involved in construction and repair of roads and bridges, building and maintaining power grids, telecommunication networks and sewage systems. Worldwide stimulus funds have been steadily injecting more money into the sector. The largest infrastructure ETF is iShares S&P Global Infrastructure Index Fund (NYSE Arca: IGF), currently up 12.9% year-to-date, with $267 million in assets.

Transportation. There’s no denying the economic impact of transport systems. The Claymore/Delta Global Shipping ETF (NYSE Arca: SEA) is currently up 25.9% year-to-date, with more than $70 million in assets. SEA is a good indicator for economic activity and commodities demand.

Green. The green sector provides environmentally-friendly technology for an evolving society. The largest green ETF is PowerShares Cleantech Portfolio (NYSE Arca: PZD), currently up 29.8% year-to-date, with $145 million in assets. PZD tracks the performance of companies whose products improve productivity while reducing consumption of natural resources.

Other. Or “miscellaneous” category that includes gaming, luxury items and Chinese real estate to name a few. The largest ETF in this area would have to be Market Vectors Gaming ETF (NYSE Arca: BJK), currently up 46.7% year-to-date, with $108 million in assets. BJK tracks global gaming companies.

Max Chen contributed to this article.

The state of Nevada recently stated that its gaming revenues have fallen to 2004 levels and gaming revenues on the Las Vegas strip fell 14.8% in June.  It wasn’t all gloom in Sin City, as for the second month in a row, high-end play increased despite the absence of special events, states Howard Stutz of The Las Vegas Review Journal.

Gaming is also picking up overseas. Many major players are looking at Macau, including Wynn (WYNN) and Las Vegas Sands (LVS), which have casinos there. Macau accounts for 61% of Wynn’s adjusted income and 69% of Las Vegas Sands’, reports Janet Morrissey for CNN Money. Macau generated $14 billion in revenue in 2008 – more than double what Las Vegas did.

It seems that Nevada isn’t the only state suffering from a decline in gambling, as eight out of the 12 states that allow commercial gambling saw declines in gambling revenues.  In addition to this, it seems that lottery revenues are shrinking as well. Fourteen out of 20 states that have lotteries witnessed revenue declines, states Conor Dougherty for The Wall Street Journal.

• Market Vectors Gaming (BJK): up 32.8% year-to-date; Las Vegas Sands is 4.7%; Wynn is 4%

For more stories on gambling and gaming, visit our gambling category.

Kevin Grewal contributed to this article.

The CityCenter is one of the largest privately financed developments in the United States. The $8.5 billion city within a city is set to open later this year. Allison Gregor for The NewYork Times says this 67-acre development on the Strip consists of six towers with 2,392 apartments, as many as 5,900 hotel rooms, a casino, a shopping center and an entertainment complex.

Some of the finer qualities of the building were tuned by the world’s best architects, and this “city” will have its own fire department and power plant. Major obstacles were hit upon building the CityCenter, as the credit fallout of 2008 stalled financing, technical difficulties were hit upon building, and the MGM Mirage, an arm of Dubai World,  had its own serious problems.

Dubai World, which had put $4.3 billion into the CityCenter project since forming the partnership in August 2007, filed a lawsuit against MGM Mirage (MGM) seeking relief from its obligations. As of April 29, the two partners had worked out an agreement whether or not bankruptcy is filed for.

This center is set to be the mecca in Las Vegas and for it to fail now would be miserable at best. Entertainment and gaming shares and ETFs should be able to benefit from the completion as well.

The gaming ETF has seen its assets surge from just $2 million in March to $37.7 today. Could investors be betting on Vegas in an economic recovery?

• Market Vectors Gaming ETF (BJK): up 19% year-to-date