If there's any other sector exchange traded fund (ETF) that gets more attention than the nano ETF, it is the biotech ETF. Again, from my perspective, these ETFs are far too focussed for my style of investing. Of course my strategy is going to differ from yours, so you may want to look in to this particular area.
Barclays has an biotech offering (IBB) that follows the NASDAQ Biotech Index and which I believe is the most popular one out there. Although interest in it, as measured by media postings, has waned recently, it still holds $1.9 billion in assets and it is probably the most diversified of the lot with 176 underlying stocks. The biggest limitation to this one is that it doesn't hold Genentech because it isn't listed on the NASDAQ.
PowerShares' offering (PBE) on the other hand has only $250 million in assets. One good thing about this ETF is that the top-ten holdings are a good mix of large and small capitalization companies.
Rounding out the list are the SPDR Biotech (XBI), the AMEX Biotech from First Trust (FBT), and the Biotech HOLDRs (BBH) which has the dubious distinction of holding the least number of stocks.
My take on sector ETFs is that as an average investor, you're more likely to mess up than not so why take the chance?