Flametouched Iron weapons are good-aligned. This means it's going to hurt evil creatures more by bypassing their damage reduction. You'll want to take this one against flesh renders, fire reavers, ice flensers and ghostly skeletons.
Even better, this gives you a bit of protection against evil outsiders in the form of a +1 to saves against damage from them. What qualifies as an evil outsider? The DDO wiki provides a handy list of enemies in this category, including named foes.
Darkwood and Densewood
You might reflexively view wooden weapons as inferior to the strength and resilience of metal weapons, but don't be so quick with these two. DDO has its share of creatures that will eat your metal weapons in no time flat; rust monsters and slimes are tough on metal weapons for obvious reasons. While Muckbane is easy to obtain and will be useful to you for quite a while, you'll want to keep an eye out for a good Darkwood or Densewood item as you move into higher levels.
So what makes these better than your average balsawood starter sword? Aside from not being as tasty to rust monsters and slimes, these two types of weapon materials have been magically treated to make them as strong as steel.
Byeshk is a word you're going to become intimately familiar with as you begin doing content that involves Beholders. Byeshk is a very hard and dense metal with the unique ability to bypass the damage reduction of the Daelkyr.
Daelkyr are native to Xoriat, and they are the barrels of fun responsible for creating Beholders. A byeshk weapon is handy to have in case you run across any Daelkyr and want to take a moment to tell them thanks for that.
Adamantine is good stuff. It bypasses the damage reduction of many creatures -- most notably constructs. You're going to come across these fairly often in the form of golems and iron defenders as well as Inevitables a little farther down the road.
This is an important one to keep handy, because the very nature of a construct may render a lot of the tools in your arsenal ineffective. Anything that would affect the mind, such as charms, fear, and so on, is useless because constructs aren't living creatures. Similarly, physical effects like poison, disease, and sleep won't work either. Their tough physical construction gives them a damage reduction that makes them difficult to hurt with melee or ranged damage, so you'll find that the extra edge given by an adamantine weapon makes a significant difference.
Cold iron is listed as "effective against fey creatures." Fey creatures are defined as things like dryads, pixies, satyrs, and so on. They are also currently nonexistent in the DDO right now. DDO community members have been requesting some new creatures, and Turbine has been moving in that direction with the last few updates, so with any luck we'll see a use for cold iron soon.
Finally, silver. Weapons composed of silver will bypass the damage reduction of vampires and devils. Those creatures, of course, are evil-aligned, so a good-aligned mod on a silver weapon will deal some serious extra damage.
There you have it -- the basics of weapon composition in Dungeons and Dragons Online! As you can see, some are handier and will be used much more often than others, but it's nice to keep one of everything in reserve just in case. Keep an eye out as you play and you'll be ready to take on anything!
Exploring Eberron is a novice's guide to the world of Dungeons and Dragons Online, found here on Massively every Friday. It's also a series of short summaries of lower-level DDO content, cleverly disguised as a diary of the adventures of OnedAwesome, Massively's DDO guild.