As many people here, I know of a few identities involving expressions of the type $\sum_{i}\ f(i)$, with "arbitrarily complicated $f(\cdot)$", as well as closed formulas in some cases. 

I also know of a few references ([Concrete Mathematics](http://www.amazon.com/Concrete-Mathematics-Foundation-Computer-Science/dp/0201558025), [A=B](http://www.math.upenn.edu/~wilf/AeqB.html)) that can help me with "hard" sums. However, is there a reference on the web that lists useful identities and related open problems (i.e. expressions for which no closed forms are known)? I wouldn't expect any single reference to be comprehensive, but perhaps there are some that I should know about?