Minecraft 1.5 "The Redstone Update" is now out. To update your Minecraft client, just launch it and click on "Yes" when prompting for update.
This new version bring a huge list of changes, improvements, new blocks:
-Engine optimizations with a more realistic lighting
-New format for texture packs, support for HD texture packs and fonts, textures can be animated!!
-Detailed death messages
-New blocks & items: activator rail, block of redstone, daylight sensor, dropper, hopper, redstone comparator, trapped chest, nether brick, nether quartz
-More powerfull mobs
-Lots of bug fixes (105 bugs fixed)
You can view the full list of changes here on the official Minecraft wiki: http://www.minecraftwiki.net/wiki/Version_history
If you are looking for 1.5 servers to play on: http://minecraft-mp.com/version/1_5/