First, my apology for this soft question. My excuse was that I really think this may be of interest to the community. I would understand if it gets closed (I have advocated closing some soft questions myself, so it's fair).
Background/Motivation: Recently at an AMS meeting I decided to attend a special session outside my interests and had a lot of fun. Also, to learn about what is going on in a different but related area, I found it very useful to look at some recent conferences and start by skimming the list of speakers and abstracts/notes. So a website with recent and upcoming conferences would be useful. They are not always easy to find with Google.
Requirements: Since this is soft, let's take some care to make sure it will not be too easy to answer. So let's define "specialized community" to be "there is at least one AMS special session devoted to (part of) it. Also, the more useful the website is the better (for example, one should preferably contains list of conferences, and perhaps more)
Examples: here is one I visit frequently: for commutative algebra
Another one I found recently and like a lot: Finite dimensional algebras
Surely there must be others?
Best Answer
Open source mathematics software -- we've had a lot of talks about Sage in special sessions, and next year in Boston we'll have an AMS minicourse on Sage.