Once you've learned one programming language or programming tool, it's pretty easy to get into another similar one. Sometimes, though, the fine differences are hard to remember. Hyperpolyglot is an awesome tool for looking up the differences and similarities between programming languages and tools.
In addition to comparing popular interpreted languages as shown above, Hyperpolyglot also covers C++ style languages, GUI scripting languages, Unix shells' commands, text editors, relationship databases, and more categories.
It's a good site to have bookmarked when it's the middle of the night and your brain is fried and you can't remember whether to write "String," "string," or "str."