Free fiction: Arthur Clarke's "Superiority", plus Eric Frank Russell, Murray Leinster, & Keith Laumer
4 old stories at Webscription, 3 of them among the most famous of science fiction: Keith Laumer's "Field Test" (download), Eric Frank Russell's "Allamagoosa" (download), Murray Leinster's "Exploration Team" (download), & Arthur Clarke's "Superiority" (download).
Two of these are among the very best of science fiction: "Superiority" & "Allamagoosa". Former is very hard to find online, & has so far been available only to US readers via Google Books. Later has been online a long time at scifi.com, but has been going on/off since they switched domain to syfy.com.
"Exploration Team" is the other very well known story.
[via Free Speculative Fiction Online]