Amid intensifying competition for ETF assets, fund providers have shown a willingness to cut costs. Several major ETF issuers now offer an array of funds with annual expense ratios of 0.1% or less.

Truly frugal investors can double their pleasure by not only purchasing low expense ratio ETFs, but also by buying those funds commission-free from several of the most prominent online brokerage firms. Still, there are issues associated with commission-free ETF trading investors should be aware of.

“Investors must be aware that many ETFs available for free trading are thinly traded and have wide bid-ask spreads, which can add to trading costs. Also, some brokers charge fees for short-term trade,” reports Trang Ho for Investor’s Business Daily.

IBD’s 2013 Best Online Brokers survey evaluated some of commission-free ETF platforms with the best customer ratings.

One of those platforms was Charles Schwab’s (NYSE: SCHW) OneSource. That platform now offers 121 ETFs on a commission-free basis after adding 16 funds to its stable in October. In addition to being able to trade all Schwab ETFs sans commission, Schwab clients can trade select funds from State Street Global Advisors, Guggenheim, PowerShares, ETF Securities and U.S. Commodity Funds commission-free. [Schwab Adds ETFs to Commission-Free Lineup]

“A $1,000 account minimum is waived if you automatically deposit $100 every month or open a checking account. An added bonus is that Schwab ETFs carry the lowest expense ratios in the industry,” according to IBD.