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Relational databases aren't cool, but they are hugely important to businesses around the world. I've been involved with databases for quite a while, in some cases actively pushing their limits.
Lately, the most interesting database I've used is Postgresql. Dumb name, great product. http://www.postgresql.org/
Oracle is big and ugly, but so are Mack trucks. Both are useful. Oracle runs on Linux, there are some products to help with this process at: http://www.orasoft.org/
SQL isn't the only way to talk to a database. Xplain is an alternative: http://www.berenddeboer.net/xplain/index.html
Documentation
This tool builds javadoc-like pages from an Oracle schema. I'm not sure that this is a good idea in general (better to generate docs from the source code), but it could be extremely useful for reverse-engineering. http://oraschemadoc.sourceforge.net/