Since I was a kid, invented languages have been near and dear to my heart; in fact, I attempted to invent my own language (with very little success) when I was young, and then I grew up to teach a course on invented languages, for which I finally invented my own language. In the spirit of sharing the love of invented languages, I am sharing these six very cool websites about or promoting invented languages:
The first five are about specific invented languages while the last link is for anyone interested in becoming a conlanger (that is, someone who invents languages (or constructs them)). There are many, many more websites about invented languages, but these are some of the most helpful for anyone interested in a specific language or in what goes on behind the scenes with inventing a language.
As I'm closing this post, I realized I focused solely on creative invented languages, but I'd be remiss if I didn't at least mention that some languages were constructed to serve as auxiliary languages, such as Esperanto. While such languages are interesting in their own right, I think the fictional languages have more life because they're meant to reflect an entire culture of a fictionalized civilization. I will also say that while I have never learned to speak any of these languages, I can spend hours learning about them metalinguistically without once getting bored.