Dragonmarks are feats that represent magical sigils that cover your body. They are from Eberron, and will likely be a rare DM that allows them outside of that world. Inside that world, they indicate a connection to the "Draconic Prophecy", which mean permission for your DM to mess with you. That's usually a good thing. Certain dragonmarks usually manifest on specific races. The Eberron Player's Guide specifically avoided giving racial requirements for these feats, but your DM might anyway. The more your DM likes the world's canon, the more likely he is to restrict them. I don't find the aberrant marks worth a feat, even for you. The effects are just too small to be that conditional for a feat. The marks of Healing and Hospitality are for leaders. The Mark of Handling is for those with mounts, which usually isn't you.
Mark of Detection (EPG,Half-Elf)
Gives you spectacular radar. Odds are you have enough perception, but maybe if your DM is difficult about passive perception.
Mark of Finding (EPG,Half-Orc or Human)
Pretending to be a better battlemind isn't a horrible trick for an off-defender: just make sure you have a nice perma-CA feat, like Deadly Draw . But remember the lament of chasers everywhere: monsters already will never run. No need to increase the disincentive.
Mark of Making (EPG,Human)
Most of the time, you are going to be limited by cash on hand as much by the "no higher than your level" cap, so the value of this is very dependent on your DM giving you plenty of cash or cash equivalents. Some of the rest of the time, you'll be limited by Rarity from making anything.
Mark of Passsage (EPG,Human)
You shift and teleport a lot, so this should be awesome. But odds are you shift plenty already, and your teleports don't generally have range limitations. If you find yourself frequently wishing you could shift more, then upgrade this to blue.
Mark of Scribing (EPG,Gnome)
Ritual Caster + Linguist for one feat. A decent deal, and if you're retributive you may not suck at rituals.
Mark of Sentinel (EPG,Human)
Your OA is usually too scary for you to get too many of them.
Mark of Shadow (EPG,Elf)
Decent for Ninjas, but not really anyone else. The Paragon path it opens up for Elf Ninjas and Chasers is quite nice though.
Mark of Storm (EPG,Half-Elf)
You have basically no thunder or lightning powers, so you don't get most of the feat. But some of you do fly, at will. If you work at it, you can use an elemental weapon or Ki-Focus to make all your attacks lightning, and forced movement enhancers to maximize the slide. Which has given the feat a bit of a reputation... because everyone should like at-will powers that slide targets 4 squares and knock them prone.
Mark of Warding (EPG,Dwarf)
This is for fighters. And Warpriests.