If you're looking for exchange-traded funds to add, the pickings just got cheaper in many cases.

Earlier this month, Vanguard shaved its fees on four of its popular ETFs. Those were:

• Growth ETF (AMEX: VUG), from 0.11% to 0.10%.
• Value ETF (AMEX: VTV), from 0.11% to 0.10%.
• Small-Cap Growth ETF (AMEX: VBK), from 0.12% to 0.11%.
• Small-Cap Value ETF (AMEX: VBR), from 0.12% to 0.11%.

Also, the new Europe Pacific ETF (AMEX: VEA) wound up the year at 0.12%. The fund opened last July and was expected to assess expenses of around 0.15%.

"We originally estimated an annualized expense ratio at higher levels," said Rebecca Cohen, a Vanguard spokesperson. "But after the year closed out, expenses wound up being less than originally estimated."

While relatively tiny moves, the latest changes further distances Vanguard's ETF lineup from the pack. It also brings to 18 the number of different ETFs that Vanguard has cut expense ratios on within the past four months.

The flurry of cost-cutting leaves Vanguard with an average expense ratio at 0.16%. Through year-end 2007, Lipper data showed an average ETF in the U.S. with an expense ratio of 0.53%.

"As ETFs grow in size, they generally become more efficient to run," said Vanguard in a statement.

As a shareholder-owned company, Vanguard says its "policy has always been to pass the savings from those efficiencies through to investors. The new expense ratios reflect the lower costs of managing these products."

Of the 16 large-cap value ETFs in IU's database, VTV remains the clear low-cost leader. In fact, the closest competitor in terms of expense ratios is Vanguard's Mega Cap 300 Value ETF (AMEX: MGK) at 0.13%. The next closest are the iShares S&P 500 Value (NYSE: IVE) at 0.18% and the iShares Russell 1000 Value (NYSE: IWD) at 0.20%.

In large-cap growth, VUG's 0.10% also faces another Vanguard fund as its closest rival. That's the Mega Cap 300 Growth ETF (AMEX: MGV) at 0.13%. The next lowest in expenses among the 13 listed are iShares S&P 500 Growth (NYSE: IVW) at 0.18% and iShares Russell 1000 Growth (NYSE: IWF) at 0.20%.

A similar cost trend shows up in small-cap ETFs. For example, Vanguard's VBR leads among 10 small-cap value funds with its nearest rivals both at 0.25% expense ratios. But the expense ratio difference widens considerably from that point.

The closest to VBR are iShares S&P 600 Small Value (NYSE: IJS) and iShares Russell 2000 Value (NYSE: IWN), both at 0.25%. But the rest of the field is spread out between the upper-30 basis points level through the mid-60s. The lone exception is the SPDR Dow Jones Wilshire Small Value ETF (AMEX: DSV) at 0.26%.

In October 2007, Vanguard reduced expenses at 10 sector- and foreign-focused ETFs. Those were:

• Consumer Discretionary ETF (AMEX: VCR), to 0.22%.
• Consumer Staples ETF (AMEX: VDC), to 0.22%.
• Energy ETF (AMEX: VDE), to 0.22%.
• Financials ETF (AMEX: VFH), to 0.22%.
• Health Care ETF (AMEX: VHT), to 0.22%.
• Industrials ETF (AMEX: VIS), to 0.22%.
• Information Technology ETF (AMEX: VGT), to 0.22%.
• Materials ETF (AMEX: VAW), to 0.22%.
• Telecommunication Services ETF (AMEX: VOX), to 0.23%.
• Utilities ETF (AMEX: VPU), to 0.22%.

In late November 2007, expense ratios at three Vanguard international ETFs were reduced. Those were:

• Emerging Markets ETF (AMEX: VWO), from 0.30% to 0.25%.
• European ETF (AMEX: VGK), from 0.18% to 0.12%.
• Pacific ETF (AMEX: VPL), from 0.18% to 0.12%.