Although railguns (cannon balls) would probably be used, missiles (they move around and could correct for gravity and evade) would still be the go to weapon just like Earth. Also, no idea why people say space battles are impractical. You can use more WMDs (antimatter) in space without as much friendly fire risk. Anti-satellite weapons exist right now.
http://en.wikipedia.org/wiki/Anti-satellite_weapon
I'd say that this would be the breakdown:
WMD, asteroids (as mentioned) smashed into planets, antimatter weapons (IPM), nukes (IPM, using tandem warhead for maximum damage against space targets), chemical weapons (injected into the hull). Nano warheads, bio-weapons
Conventional, long range: Space designed missiles carrying HE (with internal oxidizer/oxygen etc.) or probably dense kinetic warhead. Movable mines. Plasma warheads.
Conventional, medium range: Railguns, possibly modern ballista type weapons, fighter drones.
Conventional, short range: Energy weapons (better in a vacuum), probably used for missile shield as well. Static mines.