Damn, not talking about the standard-php-mysql stuff, but about new things, innovative applications and so on.
The common cliche is that most things have been developed, so there is not much left to do but reinvent. That is wrong. When google hit the search engine market, everybody was convinced that they would fail, the market was already crowded by many providers, though no one offered a simple, speedy search without a bloat interface. Nowadays, google leads.
37Signals did the same thing, although way more small-scaled. By providing easy-to-use applications for small businesses, they just created what many people need, communication-enabled platforms.
I guess the real challenge is to create things that everybody will like, yet nobody developed already. And there are many things, like an eBay replacement ( ebay sucks ), contact management, password management.. I could easily extend this list, but it's up to your imagination what to do with all this stuff.
Certainly, we don't need any more CMS or blog-engines, no guestbooks, no bulletin-boards, not even new wiki engines. They are there and working, so start doing something new. Please.