The new non-relational databases have fairly different designs. For example, if your data set would fit entirely in memory (on one or a few servers), Redis would probably be a great choice. Their different strengths come out of the design choices that set them apart.
A while ago, there were several different database query languages for relational databases, too. In interest of having a standard, they compromised on SQL. There are lots of version control systems, parsing frameworks, programming languages, etc., too. This isn't really unique to databases, they just get talked about more since there's so much buzz about hot new web development stuff.
Huh, apparently my saying so is enough to cite this as fact in kez's blog.
One good source about relational databases (including their history) is _An Introduction to Database Systems_ by C.J. Date. The author has an axe to grind, but he's thorough, and there are plenty of other references cited should you want to dig deeper.
A while ago, there were several different database query languages for relational databases, too. In interest of having a standard, they compromised on SQL. There are lots of version control systems, parsing frameworks, programming languages, etc., too. This isn't really unique to databases, they just get talked about more since there's so much buzz about hot new web development stuff.