Harry Potter readers may hate the name, but Project Voldemort is a "big, distributed, persistent, fault-tolerant hash table." You put in key-value pairs and they get serialized to disk somewhere in a cloud. There are no queries. You can't ask for fields within a certain range. You just ask for a key and get the result. When data is stored to two different nodes at once, as might happen in an ACID-free environment like this, Voldemort uses a vector clock to choose a winner. It's very, very lightweight. You might even try to make a joke about the real Voldemort's shadow-like presence.
HBase and Hypertable are two projects that aim to imitate Google's BigTable. HBase was built in Java on top of Hadoop and sports a C++ API. Hypertable is written in C++ and uses a C++ API.
A number of online services wrap a nice Web-based interface around a database. Some call themselves "shared spreadsheets," but they are just databases at their core. These SaaS offerings, including Caspio Bridge ( see my review), DabbleDB.com, and Zoho (Zoho Sheet and Zoho Creator), are mainly targeted at businesspeople, not programmers, but they can be useful.
And if you don't want to leave relational databases behind forever, there's always Drizzle, a fork built by some of the MySQL team. The new version built under the GNU Public License tosses out many of the fancy features that MySQL tried so hard to incorporate. "Stored Procedures, Views, Triggers, Query Cache, and Prepared Statements are gone for now," wrote Brian Aker last year. Now, there are dozens of branches in the MySQL tree. Oh well. Once it was trendy to have too many features. Now it's not.