A type of stock offering traditionally used for small share sales has morphed into monstrous proportions this year, thanks in large part to capital-hungry banks.

At-the-market offerings, or ATMs, have been deployed for exponentially larger amounts than in the past, and this month banks seeking to raise capital have been leaning on them hard.

The deals, under normal conditions, allowed companies to register stock that they would sell into the market bit by bit, usually at no more than 15% of their total daily volume. Also known as "dribbles" for the gradual way sales occurred, they were considered a flexible way to tap the market when conditions seemed best.

Today's dribbles look more like a release from the Hoover Dam. Bank of America Corp. (BAC) grabbed headlines earlier this week by selling 825 million shares in one day under its ATM program, more than half the total $13.5 billion it moved onto the market since it began "dribbling" earlier this month.

Last week, SunTrust Banks (STI) registered to sell $1.25 billion through the ATM process. Four other banks have also registered for dribbles this month, in amounts ranging from $350 million to $750 million, according to data from Dealogic: Fifth Third Bank (FITB), PNC Financial Services Group Inc. (PNC), KeyCorp (KEY) and Marshall & Isley Corp. (MI).

Banks are the most recent issuers to line up for ATMs, but a they aren't the only sector deploying them this year. Transportation companies, utilities, and real estate investment trusts - all typical dribble issuers in the past - are also using them more, but generally at smaller sizes of $300 million or less.

The number of ATM offerings so far in 2009 is trending more than three times the level seen at this point in 2008. With 42 filed since the start of the year, they will easily outpace the 57 seen throughout all of 2008, by Dealogic's count.

-By Lynn Cowan, Dow Jones Newswires; 301-270-0323; lynn.cowan@dowjones.com