Thin lawn with crabgrass.
 

My lawn is not very dense and there is quite a bit crabgrass as well. Wonder what can be done to dense it up and get rid of the crabgrass. I have tried overseeding but I dont see it helping much.
Is there any particular crabgrass killer and fertilizer to help me get ahead on this issue?

Crabgrass
 

The best way to control crabgrass is with a pre-emergent control and now is about the time to apply it. Crabgrass grows every year from seed left by the previous years crop. Be aware that the pre-emergent will also kill off grass seed at the same time so this is a multi-step process. You can't kill the crabgrass and re-seed at the same time. There are MANY brands of crab grass controls. Head to your local garden center (not a Lowes/Home Depot) and ask a couple of questions. They will get you hooked up. Best to go sooner rather than later......

OK, so I did get fertilizer with crab grass germination control which is probably what doechsli was talking about.
Is the crabgrass from previous year going to die off then? Not sure if I am going to have to treat the crabgrass spots with something like Total WipeOut or not.
It looks like I will also have to overseed probably 2 - 3 weeks after I apply the crabgrass control stuff, what seed is recommended. I am asking because I don't see too much improvement when I am using the regular seed and I wonder if stuff like the ez-seed is worth the buck?

Crabgrass is an annual. As doechsli pointed out it grows every year from last years seed. That is why the pre-emergent product works to control it. As he also pointed it will also kill any new desirable seed you put down so read the bag carefully to see how long you must wait before you overseed.
As far as what seed to use, do you know what primary type of grass you have now? If not then you should talk to your local lawn people about what they recomend for your area. Also consider the type of use the area you are seeding gets. Do you have kids or lots of traffic in this area? Is it bright sun all the time? Or is it partial to full shade, etc? These are considerations which will determine the type of seed best suited for your yard.