"Everyone Knows" ...

• ... that c is widely used for low level programs in large part because operating system/device apis are usually in c.
• ... that Java is widely used for enterprise applications in large part because of enterprise libraries and ide support.
• ... that ruby is widely used for webapps thanks in large part because of rails and its library ecosytem

But lets go into to details what are the specific niches and subniches. Especially with respect to libraries. Where might you embed lua for application scripting versus python. Where would you use Java vs C#. Which languages do different scientists use? Also which languages have libraries for these subniches? Things like bioperl/scipy/Incanter.

Please no flamewars about how nice each language or environment is. This is where they used. Also no complaints about marketing/PHBs.

(Manually migrated) I asked this question again after it was closed on stackoverflow.com