Again, alot happened behind the scenes. Maybe you have noticed it already, there was a version change. Oy broke 0.5.2 compatibility as another step to 0.6.0. You can download the newest versions here. (Make sure to always have the latest version, since the static netversion was removed so you can't connect to servers that have a newer version)
What does that mean?
You can't connect to 0.6 trunk servers with a 0.5 version and vice versa. This is a nice step for developers, because they don't have to care about backward-compatibility anymore. Many nice things like country flags for players or a clan tag seperated from the player's name have been added.

Changes that were only possible because of the compatibility break, are marked with green.
Really important changes are bold.
Gameplay changes are red.

Changes since the compatibility break :

• Changed the netversion


• Added IPv6 support


• Added friend support in server browser


• Added clan name support


• Added possibility to set your country flag (It will show up next to your player name)


• Added new emoticons by Landil (link)


• Added aliases for votes


• Fixed demo recording on the server


• Added force_vote and remove_vote command (with menu)


• Added spec-vote (vote for making someone a spectator)


• Reworked ninja - it won't steal your speed anymore


• Added chat history function (ability to send old messages again)


• Added new entities by Landil


• Added sound for highlighted chat messages (chat messages that contain your name)


• Added ability to ignore some players' chat


• Added ability to hide the hud

Rather uninteresting changes (e.g. fixes)

• Added ability to follow players in spectator mode


• Added extended demo info


• Replaced team name by clan name if possible


• Removed the progression info from the server info


• Added notification when closing the editor/teeworlds when there is unsaved data in the editor.


• Added high-definition icon for the client and added an icon for the server


• Enabled debug rendering in the editor


• Fixed the map downloading for map changes


• Fixed the quick search on maps, so the browser shows the match


• Made the client look for maps in subfolders


• Made the sound load threaded again


• Made the server show disconnect reasons, e.g. "*** nameless tee has left the game (Timeout)"


• Security: Made the client check whether version server and master server packets really come from these servers


• Made version change notice appear in red color


• Show the console's current page number when scrolling backwards

Big thanks to Kottizen. :)
Thanks to Choupom, he spotted some weird things in the list.