Last month, the game makers at Blizzard announced a remastered, 4K-friendly version of the original StarCraft, set to launch this summer. That announcement also teased a much sooner release of the original StarCraft that would be completely free. After a delay, that free version is finally available to download worldwide. Blizzard has posted links to Mac and Windows builds here.
The late-March remaster announcement included the promise of the game's original, lower-res version, complete with all Brood War expansion content. The low-res version would be made entirely free to download (technically, "free as in beer"—you can grab the compiled game and play it to your heart's content, but you are not formally allowed to pick the files apart and use, say, the game's source code as you please). However, a beta launch of this new version, numbered 1.18, resulted in crashes and issues for testers, so Blizzard held it back for testing and tweaking until Tuesday.
It makes sense that the patch's beta launch was a little rocky, considering the game hadn't been updated since 2009 and had to pass muster on modern Windows and Mac operating systems. This version may very well receive further patches as well, since it will contain identical gameplay to the summer remaster version. The newer StarCraft will only receive a superficial update in the form of completely redrawn assets; all other parts and mechanics of the game will be so identical that remaster players will be able to compete online against original-version players.