I want to build/learn something this weekend - suggestions?
DocSavage: Try building a program on Google App Engine.  I personally like python, but you could go with any JVM-targeted language.  The particularly new part for you is the datastore:http://code.google.com/appengine/docs/python/datastore/No joins (except for "self joins"), entity groups, sharded counters, and the mindset that computations should be on write instead of on query.  At the end, you can work through this article that shows how to implement a YC-style decaying vote score purely on writing of entities (records):http://code.google.com/appengine/articles/overheard.html