I'm sure most have heard that ranged combat is very weak in DDO. That doesn't mean its useless, at some point in time every character is going to want to have some form of ranged combat to get rid of a sniper perched far away that you cant reach. So, what do you do? which weapon is best?
on your character sheet look at your STR score, next to it is the bonus number, if its positive then you have high STR and want to milk that bonus. if its negative then you have low STR and want to avoid a penalty. if its 0 then your in the middle.
if you have low STR then the cross bow is the weapon for you. it has no STR bonus or penalty. the heavy xbow does the most damage unless you get one of the exotic xbows. the exotic xbows cost feats and are only worth considering if your build has a few spare feats.
if you have silly low STR and keep getting encumbered (loose AC and will down very fast, check the load bar in your inventory) then maybe a throwing dart or shuriken would be better. halflings tend to suffer from this if STR isnt important to their build and also get a bonus to throwing weapons to off set the STR penalty to damage. but get a STR item fast, running back to town to sell every suit of full plate you loot is a real pain in the sitting tool, trust me, i have a base 7 STR halfing
if you have high STR then things get a bit more complicated.
if you have 2 levels of ranger then use a long bow. rangers get bow strength and rapid shot for free. bow strength means you can add your STR bonus to the damage, rapid shot means the bow fires at a half decent rate.
if you have martial weapons but no bow strength then use a throwing weapon like the throwing axe. this allows you to add your STR bonus to the damage without needing a feat. keep an eye out for a returning throwing weapon.
if your don't have either, then its back to the cross bow unless your race/class gets a different type or ranged weapon.
if your on 0 STR bonus but can use any of the above then just go with the best thing you loot
if your can't close into melee range quickly then its the right time to use a ranged weapon. this can happen if the mobs are some where hard to get, or if a squishy is running round in a blind panic with a kobold chewing on its leg and you're getting fed up with the benny hill music. please note in benny hill chase scenes you wont hit the mobs if they are running at an angle, so place your self somewhere the squishie is going to be running straight at you.
there are also some levers you need to hit with a ranged weapon, sometimes there is a chest near by with a handy bow but its best to get prepared any way.
also if you have many shot then there are more reasons to use ranged. if you have a clear line to a caster before a fight starts then many shot might be able to kill him before he becomes a danger. once dead whip out those blades and wade in to the rest of the fight. if you got many shot from free ranger feats then you will also get improved precise shot, this means you don't need a clear line of sight on the caster to hit him with many shot thanks to precise shot. if you turn improved precisse shot on then you may also hit every thing in between you and the caster. thats good if you can handle the aggro, but bad if your a squishy
one more trick with many shot, later on in the game you'll meet beholders. if you managed to get your grubby mits on a weakening bow and have 3 or more arrows in each many shot then you can drop that beholders STR to 0 giving you and everyone else auto-crits! nothing drops faster than a beholder being wholloped by a raged barbarian with a great axe getting auto-crits. and your bow will be adding a lot of damage too when its auto-crittin.
when your taking out a ranged mob you can dodge most of its attacks. when you see it fire, side step and its attack will go sailing past you. this is very easy at long ranges but harder at short ranges. at short range its better to duck behind some cover, this allows you to spend the time reloading your shot in safety, then step out from cover to take your shot before you go back into cover. one excellent example of this is the 3 casters above the first acid pit in "Proof is in the Poison" quest. when you move out to take your shot you can stop for a fraction of a second before firing to avoid the -4 movement penalty, takes a bit more practice but is very helpful if you don't have a high dex.
see sirdanile's post below for more details on using ranged attacks as your main combat style