Yesterday Mojang announced the first Minecraft snapshot for Minecraft 1.7, called 13w36. This snapshot might very well be the biggest one yet, with lots of new features and stability bug fixes.The biggest improvements are the new biomes. These include Clay Canyon (aka Mesa), a Savannah Biome, Black Forest (with some large mushrooms that grow too!), Ice Plains Spikes, Birch Forest, Flower Forest, Sunflower Plains, Deep Oceans biome, Gravel Beaches, and Extreme Hills biome.
The second main improvement is fishing. Fishing has been completely revamped in this snapshot, with more rewards (some good, some bad), new enchantments for fishing rods, and new fish (pufferfish, raw salmon, and clownfish).
There are hundreds of other small improvements, such as melons growing in forests, a rewrite of the achievements system, new ground covers, new two block-high flowers and tall grass, cloning books, creepers explode if you light them with flint and steel (similar to Minecraft PE 0.7.4), new options menu, and a lot more.
This is probably the biggest snapshot ever. Are you excited? What's your favorite addition? Tell us in the comments! And make sure to check the second source link for the full changelog.
Source: Mojang, Minecraft Wiki