Among other interesting changes:

• support for slf4j (although we currently don't configure FreeMarker's logging, we could start doing it now)
• non-automatic flush of the output: until now, FreeMarker was automatically flushing the output after each Template.process(). This is still the default behavior, but can now be changed. (see changelog for details)
• a new simple way to completely disable caching (could be interesting during development)
• finally, better tld file location mechanisms. This could be the end of the workarounds developers had to use while running from their IDE.

http://freemarker.org/docs/versions_2_3_17.html
http://freemarker.org/docs/versions_2_3_18.html