The first major step into the road of releasing 0.6.0, which will give you loads of new features, like newhouses, newindustries, signals and diagonal tracks under bridges, trams, autoslope, oneway roads, half tile slopes and much more. It furthermore contains quite a few performance improvements under certain conditions as well as a very long list of bugreports. If you are really interested, you should read the changelog.
We OpenTTD developers hope that you will thoroughly test it so we can quickly release the real 0.6.0.

Have fun!

Thread on the forum about 0.6.0-beta1.