There was a time--not so long ago -- when all personal computer software was free.

In the early days of the microcomputer, when personal computers were still the province of a tekkie avant-garde and programs were all written by a small community of dedicated hackers, it was considered bad form to charge another computer buff for a program. The software industry adhered fairly strictly to the Marxist dictum of "From each according to his programming ability, to each according to his computing need."

Then a brilliant young capitalist named Bill Gates wrote an excellent program -- a microcomputer implementation of the BASIC language. Gates infuriated many of his fellow hackers by charging money for his program. But BASIC was a valuable commodity, and many users were willing to pay.

Today, of course, Bill Gates is the head honcho of MicroSoft, the most powerful force in the $4 billion personal computer software business. And today it is an accepted fact that software costs lots of money.

But there are some places where the industry's communistic origins are still honored. For the computer user who knows how to find them, there are literally thousands of programs floating around for free.

Many people believe (do you suppose this information might come from software stores?) that you have to be a high-tech wizard to acquire and use the free, or "public domain," software. But it's not so. A lot of this stuff is readily available even to a rank neophyte.

Granted, some of this stuff is junk -- but then, so is some of the stuff the software companies expect you to pay for. There are, however, countless free programs -- utilities, games, spreadsheets, word processors, etc. -- that work well. The public-domain "Modem7" package does more than some communications software costing $150. The free LOGO language for the IBM-PC is just about as good as any LOGO you can buy.

There are many sources for this "freeware." We'll give you now a quick look at four of them, arranged in ascending order of expense.

* User Groups: If you show up at the next meeting of your local Apple or CP/M or Kaypro or whatever user group with a couple of formatted blank discs, you can fill those discs with all the software you like from the group's public-domain library.

Just about all user groups have big collections of free programs. The Denver Heath-Zenith group, for example, has about 900 programs available for the asking. Maybe only 10 percent of them are really valuable -- but that still means 90 good pieces of software you can have for free.

* BBS: That's an acronym for "Bulletin Board System," which is a free dial-up data base available to any computer owner who has a modem. A modem costs $80 to $400, but after that everything is free.

There are a dozen or more BBS systems operating now in just about every big city. To find the numbers, you might ask a user group or your computer store, or look in magazines such as Computer Shopper, Sextant, or PC World. A paperback called "The Computer Phone Book" (New American Library, $9.95) lists hundreds of them.

You hook up your modem, dial the BBS number, and then follow the instructions that will show up on your screen to track down free programs that are available.

Once you find the program(s) you want, you push the keys that start the "download" operation. The software will load from the BSS system into a disc in your computer.

All these bulletin boards have a system operator (called the "Sysop") who can answer questions if you get stuck. Sometimes you talk to Sysop by using a "Chat" command right on the data base; sometimes you can get to Sysop in a low-tech mode (i.e., call him or her on the telephone).

* Commercial data bases: The "online" operations such as CompuServe and The Source all have big libraries of free software, normally organized by type of computer -- that is, if you're looking for a free TRS Model 100 spreadsheet program, you go to the Model 100 corner of the data base and poke around.

Hit the "download" key and the program will be sucked down into your disc. This option means a little more expense than a BBS because you pay for your time on CompuServe, etc.

* Merchants: It seems almost a contradiction in terms, but there are places that sell "free" software. They charge a minimal fee (often $6 for a whole discful of programs) and distribute by mail.

There are lists of these outfits in Alfred Glossbrenner's new book "How to Get Free Software" (St. Martin's, $14.95). Two big ones, which will send you a catalogue, are PC Software Interest Group, 1556 Halford, Santa Clara, Calif. 95051 and, for CP/M programs, SIG/M, Box 97, Iselin, N.J. 08830.