A simple mathematical parser written in Go on the day before my gynaecology finals.
Build by go build -o gocal calc.go and move the binary gocal into /usr/local/bin.
gocal -i starts an interactive prompt, gocal -c takes a string as a command line argument and parses it.
Running just gocal waits for an input. BTW, the parser doesn't just compute the answer, it also shows the generated AST (abstract syntax tree).