The phenomenal growth of the personal computer as a business tool has made the PC software industry one of the most profitable, competitive and fastest growing in the world. Trade magazines are thick with advertising for software, virtually all of it aimed at the lucrative business market.

In this atmosphere, the network of hobbyists, moonlighters and part-time programmers who started the whole PC revolution may have been eclipsed, but their activity continues unabated, and signs of it appear in the most surprising places.

A striking new example is something called the $5 Computer Software Store, which is not a store at all but a new mass-marketing scheme for "shareware." That term refers to computer programs that can be obtained at no cost, or at a nominal charge, and can be tried and used freely. Shareware was invented by computer hackers for other computer hackers.

The $5 Computer Software Store is a new wrinkle -- a cardboard display stand for convenience stores, airport newsstands and other nontraditional outlets. The stands offer, on 5 1/4-inch floppy disks, a large variety of shareware programs -- from full-scale business applications to games and utilities. It is basically the same selection available from mail-order clearinghouses and computer bulletin boards.

There's a small label that attaches each disk to the back of the sleeve. Users are guaranteed that the disk is virus-free, as long as that sticker has not been tampered with. Programs that would otherwise be too large to fit on a single disk are in compressed form. Installation instructions are given by inserting the disk into a floppy drive and entering the command "Wiz."

You'll also get a brief explanation of how the shareware (sometimes called "freeware") concept works. The $5 you pay for your disk is just a distribution fee. If you try to use the programs, you are asked to become a registered user by sending a fee, which is invariably far less than the cost of similar software sold through standard commercial channels. Registered users typically are entitled to bound copies of the instructions, technical help on the telephone, notices of upgrades and other such benefits.

Computer programs have an inherent ability to be copied. Shareware takes advantage of this and uses it as a means of distribution. Most shareware authors encourage users to make copies of their programs and pass them around. Such software can also be "downloaded" by modem from computer bulletin boards and on-line databases such as Compuserve and America Online. Shareware also is marketed through mail-order houses that charge a small fee, plus shipping costs.

The shareware concept is consistent with the "hacker ethic," the spirit of openness and desire to spread computer technology to the masses, that helped give rise to the PC revolution in the first place. Shareware has made fortunes for such programmers as Bob Wallace, author of the word processor PC-Write, and Jim Button, author of the flatfile database PC-File.

Other shareware programmers haven't prospered because not everyone who uses the software is honest enough to register and pay the fee. But authors understand that the only way they lose anything when people use their programs without paying is if those users would have paid for the program if they had not been able to acquire it for free.

The authors of ProComm Plus, the modem communication program, have switched to traditional commercial channels, but thousands of users still work with an earlier version, the shareware classic, ProComm 2.4.2, and it is widely available through shareware catalogues and libraries.

ProComm is but one of several shareware communications programs that have become major hits. Another is Unicom, a fully featured program written for Microsoft Windows that gives any commercial Windows modem program serious competition. Modem software has always been a specialty of shareware authors, since most of them are hackers and make heavy use of their modems.

There is now a growing selection of useful Windows shareware, much of it utility packages designed to accomplish such tasks as enlarging the arrow mouse pointer and streamlining the program's file management capability, which ought to be a snap in the Windows atmosphere, but isn't.

Shareware has always been an excellent option for anyone in need of software, but the process of getting it has been a bit cumbersome for those not accustomed to using a modem, or sending away to an obscure mail-order house. What's promising about the $5 Computer Software Store is that it puts the software where it's easy to buy, and at $5 a disk, not much of a risk to try.

Brit Hume is a contributor to the Washington Post Writers Group. He is chief ABC News White House correspondent and the founding editor of a computer newsletter.