Want free credit? National Australia Bank is the bank to ask. It will charge no interest all on its personal credit cards for 15 months.Citi-funded Virgin Money used to have the edge when it came to aggressive offers on cards, with 14 months free money offered.Before that, Commonwealth Bank had the edge in product risk, with a 12 month interest-free offer.NAB has no doubt noticed the huge gains in market share made by Commonwealth in credit cards and has also lowered its rates on personal loans.MWE Consulting's analysis of Reserve Bank of Australia data puts Commonwealth's market share at 22.3 per cent, or 1.4 percentage points higher than two years ago.Personal credit cards constitute a A$45 billion segment, one that earns banks annual fees and merchant fees, and the apparently optional revenue line of interest income.Commonwealth has embarked on further conquest of this market on the merchant side, driven by IT innovation, so it makes sense to grow the borrower side of the cards business at a similar tempo.But NAB wants to reclaim some of the share of this profit pool, a reminder that adjustments in market share between oligopolists do bring vitality to the retail banking market.Combined with the obvious accommodation made by banks over recent months for the rising demand for investment home loans, a considered effort is now underway to stimulate credit demand by loosening credit terms.