I think the next huge successful web application will be a blog only search engine. There is so much content in all the blogs out there. People write about their lives, they write about their jobs, they write about current events, everything is out there. Now I know that it's not all accurate, but it is certainly important information and it's virturally untapped.
For instance, I talked about that water balloon gun the other day. Think of the case where someone wants to buy one. If they are lucky, they may find the company that makes the one I bought (the best I've seen out there, but not very well advertized in the search engines).. anyway, say they find the company. Wouldn't they want to know some peoples opionions on the gun? There may be reviews on the company web site but how non-partial will they be? Somewhere in the blogsphere there are entries about water balloon guns.. but they may never be found.
This is just one example, I could come up with dozens. I know there are aggrigators and some search engines out there for blogs but nothing has been created yet that searches all blogs in a topical manner. Simple subject line searching would be useful and even that doesn't exist yet there is an enourmous amount of useful content out there.
Someone will create this engine and it will be huge. And there need not be a monetary reward for it. Yahoo and Google weren't created for profit. Someone with programming skills and vision can easily build this thing. I'd do it myself but I'm way too lazy.