Here's the scoop from PHP.net:

The PHP developers are proud to announce the immediate availability of PHP 4.3.3. This release contains a large number of bug fixes and we strongly recommend that all users of PHP upgrade to this version. Full list of fixes can be found in the NEWS file.

There are some pretty cool upgrades and features that are changed in this release including: Improvements to the IMAP extension, upgrading of the bundled Expat library to version 1.95.6, upgrade of the bundled GD library to version GD 2.0.15, and, of course, over 100 different bug fixes.

So, what are you wating for - get over there and upgrade!