With stock trading, there seems to be an app, of course, for just about anything.

Just getting started?

iTrade from Andrew Farley lets you try out ideas on a simulator based on a platform from, what else?, NerdTrade.com.

Already smart? Wall Street Scanner lets you analyze stock market data, news and social media from more than five million websites in real time. You can forecast stock market movements, based on these, just like the pros. The cost? Nothing. It's free.

E*Trade and TD Ameritrade, of course, also give you apps for free that keep you in touch, anywhere you go, with your personal trading accounts. They make their money on commissions.

Even Merrill Lynch now tries to serve its "self-directed" and advisory center customers with the Edge, for iPhone. It lets them "trade stocks, ETFs, mutual funds and options while you're on the go" and check status of orders.

But the best idea of recent vintage comes from a mutual fund company, Putnam Investments (see "Shop. Save. Invest With iPhone App From Putnam," page one).

Its Putnam PriceCheck & Save iPhone app takes the natural evolution of mobile apps to an unexpected but wholly logical conclusion that other financial services firms would be smart to copy. Particularly the E*Trades and Merrill Lynches, which want to spur more trading.

[Not a bad idea, now that trading volumes in the United States are down 45.5% versus a year ago, according to NYSE Euronext.]

The conclusion: Show people who love to shop how to save money. Then get them to invest the savings. On the spot.

Here's how it works.

Your recreational shopper and reluctant saver walks into Target. He or she points an iPhone at the object of his or her desire, say a new flat-screen television with Netflix built-in.

The phone scans the bar code and displays alternate sources of the same product on screen, from across the Web.

Then, it calculates the savings, if you proceed to order it from one of the alternate sites.

Finally, it asks-naturally-if you want to invest the savings that you just "made" in a Putnam fund in the 401(k) plan that you got through your employer.

No sweat savings. No sweat investing. With "found" money. While shopping.

The extensions are obvious.

Bank of America's counterpart app asks you if you want to transfer the same amount from your checking to your savings or home mortgage account.

BlackRock's counterpart asks if you want to put the amount into a rolling ETF purchase plan.

Morgan Stanley Smith Barney's app asks customers to put the savings into a managed savings account.

Goldman Sachs' app asks you if you want to put your savings plus a million bucks of your net worth into Facebook shares.

In any case, the principle is clear. Turn every spending opportunity into a savings, investing and/or trading opportunity.

What are you waiting for? Make your app for that, today.

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