My personal favorite is Oracle Express Edition (XE), the free version of the world's best relational database.
Here's what's great about it:
- Available for both Windows and Linux at the OTN website
- Easy to install (InstallShield on Windows, rpm package on Linux), only takes a few minutes
- Lightweight, requires very little resources to run
- Great performance
- Comes preinstalled with APEX on it
- It's free for any usage, including production applications
Oracle Express Edition is currently available in version 11gR2 (11.2). Based on hints dropped by various folks at Oracle, there will be a 12c version of Oracle XE, but it is probably a year or two away yet.
The XE documentation is a great starting point for those new to Oracle, with several "2-day guides" to database administration and development tasks.
Enjoy Oracle XE and #ThanksOTN for this great product! :-)