NoSQL refers to non-relational data stores that break with the history of relational databases and ACID guarantees. Popular open source NoSQL data stores include:

• Cassandra (tabular, written in Java, used by Facebook, Twitter, Digg, Rackspace, Mahalo and Reddit)
• CouchDB (document, written in Erlang, used by Engine Yard and BBC)
• Dynomite (key-value, written in C++, used by Powerset)
• HBase (key-value, written in Java, used by Bing)
• Hypertable (tabular, written in C++, used by Baidu)
• Kai (key-value, written in Erlang)
• MemcacheDB (key-value, written in C, used by Reddit)
• MongoDB (document, written in C++, used by Sourceforge, Github, Electronic Arts and NY Times)
• Neo4j (graph, written in Java, used by Swedish Universities)
• Project Voldemort (key-value, written in Java, used by LinkedIn)
• Redis (key-value, written in C, used by Engine Yard, Github and Craigslist)
• Riak (key-value, written in Erlang, used by Comcast and Mochi Media)
• Ringo (key-value, written in Erlang, used by Nokia)
• Scalaris (key-value, written in Erlang, used by OnScale)
• ThruDB (document, written in C++, used by JunkDepot.com)
• Tokyo Cabinet/Tokyo Tyrant (key-value, written in C, used by Mixi.jp (Japanese social networking site))

I'd like to know about specific problems you - the SO reader - have solved using data stores and what NoSQL data store you used.

Questions:

• What scalability problems have you used NoSQL data stores to solve?
• What NoSQL data store did you use?
• What database did you use before switching to a NoSQL data store?

I'm looking for first-hand experiences, so please do not answer unless you have that.