Wikis. Who woulda thunk that a website that anybody can edit would actually work, let alone become an important component of the web and most corporate intranets.

HTTP. The protocol is not that complicated, but it's become ubiquitous, and a cornerstone of the web.

Social news. Nowadays everybody takes links you can vote on as a "duh" feature and wonder what the big deal behind Reddit is, but when it came out in 2005, it was very much a "Why has nobody thought of this before? Oh, right, because nobody will ever use it" invention.