This is the newest version of KDE and hopefully the last before KDE 4 :)
This release include LOTS of bugfixes and a few minor improvements.
Here is the official release anouncement:
This release includes a number of bugfixes for KHTML, Kate, the kicker, ksysguard and lots of other applications. Significant features include additional support for compiz as a window manager with kicker, session management browser tabs for Akregator, templating for KMail messages, and new summary menus for Kontact making it easier to work with your appointments and to-do's. Translations continue as well, with Galician translations nearly doubling to 78%.
Changelog: KDE 3.5.6
Download: KDE 3.5.6