Kerbal is now Beta, and this is a sizable update.

Some major changes include: (Quoted from KSP's Blog)

A Retooled Craft Editor: Build crafts better than ever before with advanced part sorting and construction gizmos that allow players to place, offset and rotate the different parts on your craft.
Expanded Contracts: Brian “Arsonide” Provan, creator of the highly rated mod, “Fine Print,” has implemented and expanded his mod into the game, granting greater variety, depth and difficulty to the previously implemented contract system.
Biomes Everywhere: KSP-TV host, Alyson “Tanuki Chau” Young created new biomes for the game, bringing them to over 100 areas from which Kerbals can collect, store and send science data. The new biomes have been placed all across the universe, giving players bigger incentive to explore every inch of the game.
New Mk3 SpacePlane parts: Prolific modding community member, Christopher “Porkjet” Thuersam has overhauled and added to the popular collection of Mk3 parts that allow crafts to carry larger payloads.

In a surprise move the ESA (European Space Agency) has allowed Squad to use their logo and imagery. So now players can run their own Rosetta like Missions.

With .90 KSP is getting ever nearer to hitting 1.0, so here's hopping that we will start to see some of the multiplayer aspects in the near future.

For a full list of 0.90 patch notes, check out the KSP Wiki: http://wiki.kerbalspaceprogram.com/wiki/0.90