For those who haven't met XE before, it is the free edition of Oracle.
It allows for up to 11Gb of data (up from 4Gb in the old XE) and will use 1 CPU and 1 GB of RAM. The server can have more power, but it won't get used by the database.
You can use it for free (developing, testing, running production applications and even give training using it).
You don't get some of the snazzy Enterprise Edition features. No FLASHBACK DATABASE or FLASHBACK TABLE. No PL/SQL Function Result Cache. No Diagnostic or Tuning packs. Or table compression.
The big 'Standard Edition' feature I can see that is missing is Java in the database.
It comes with Apex 4.0 pre-installed.