There's a 15w31c snapshot server running at snapshot.lost-nations.net. Snapshot servers are development version of vanilla(regular) minecraft that Mojang releases. It's a development version, you should expect to be buggy! Don't build anything you want to keep. The map could be reset at any time! It's just to check out the latest blocks, items, and mobs. To use a snapshot version check the box next to "Enable experimental development versions" in step 2 of this guide. Then select the version named "snapshot 15w31c".