If you're looking to get your feet wet in the world of Oracle, then Oracle XE is for you.
Somewhat comparable to SQL Server Express Edition, Oracle 10g XE is the free version of Oracle-- free to develop, free to deploy, free to distribute. It's limited to one database per machine, 4 GB per database, 1 GB of memory, and one processor (you can install it on multi-processor machines but it will only use one).
Combined with Oracle Application Express (APEX) -- a free toolkit for building applications, including web-based user interfaces, reporting, security, and dashboards -- you can actually build little web-based database applications pretty quickly without much learning.
I think the hope is that smaller organizations will consider using it for internal applications and then grow into the more expensive versions. I'm not sure how likely that is, but I am working with a friend to build a small inventory/billing app for her small business, and I was going to use MS Access just because it's fast and familiar. After installing Oracle XE, I'm considering using it, although admittedly I'd be doing so just as an excuse to learn a new skillset.
Oracle has a reputation of being somewhat of a monster to install, and if you've ever read anything about the Universal Installer, you may think you need a PhD just to get it installed. Oracle XE and APEX are quick installs that ask you a handful of questions (install path, etc) and for the most part you can sail right through with the defaults. I had to change the default port and still don't quite know why, but even that was easy-- the installer told me that 8080 was in use, so I tried 8081, then 8082, until I found one that was open. That was the biggest glitch I ran into.
The other great thing about the XE/APEX is the tutorials available on both web sites. Oracle has created a "2 Day Developer Guide" that walks you through the tools, basic PL/SQL, and everything you need to know to get a quick and dirty app up. There's also some great tutorials.
I don't expect this will make anyone an expert at Oracle, but for SQL Server types like myself looking to teach themselves PL/SQL and maybe create a little sample app, you can at least get yourself some exposure to the world of Oracle without a lot of hassle.