JabRef

It might be a little difficult at first to comprehend, but if you are looking for BibTex entries, there's no better place to do it. You can download complete texts or search for a pattern, classify entries and much more ... Even more notable is the extensive format selection it supports and several languages being able which in my perspective puts it to a leader position.

I can't say there is anything in particular troubling me - for an open source solution it builds the way even for proprietary ones! In case you are not a Java fan though, then you might not be so attracted by this solution but at this moment I can't find anything being wrong with it.

JabRef is a professional tool in case you are working with BibTex format, at the same time though the flexibility of the software proves it's more than a one-way kind of a tool. As it is open source you might find it is actually far better developed and richer in functionality than many similar solutions.