Given that it's a .jp subdomain, it's likely geared towards Japanese. Denisu is a valid shortening of DNS - the characters to spell it in katakana would be デニス, and potentially easier to remember for Japanese than "whatismyip," a fully qualified english query. Unless someone out there who knows more about Japanese than I do (admittedly, very little) can confirm that デニス or でにす is an actual word (google translate detected でにす as a phrase, giving me "su to the," which makes sense grammatically..)
Funny enough, it's also the romanjification of "dennis" (one of the first hits on google for it is dennis rodman's japanese wikipedia article).
Uh, no? Knowing your public IP is valuable information sometimes and you need an external source to tell you that. Knowing your machine's IP is not something you should go to a website to look up, just check your local config.
I've always been partial to http://ipchicken.com . It's got commercial motives, but it's easy to spell, it's easy to ask people to go to it and have them remember, and the numbers are giant and readable. This is all good!
Yes, I agree, especially for shell scripts and such. A service should give me 1) plain text, 2) key value pairs, 3) json, and 4) human readable/copyable output.