Arlington has a map/list of providers here:
http://batchgeo.com/map/4ba82ff8bd161fde88e5e15dd1c199b1
Not all of those are for kids under the age of three, they used to have a more succinct list with ages but I couldn't find it.
If you are looking for next September, then you have plenty of time to get on waiting lists. I would start calling now -- the waiting lists for most centers in Arlington are going to be over a year. I used to live in South Arlington and I work in North Arlington, so I have toured most centers in the area. There are a lot of things to consider when looking at places -- I am sure people will have lots of feedback for specific places once you tour them.
Take the advice you get on this board worth a grain of salt though -- every place my children have had care from has horrible reviews according to this message board, which obviously I do not agree with. Sometimes people just write stuff because it's nice to vent on an anonymous board especially since with child care most parents are afraid to say anything that could be traced back to them. Just don't believe everything you read, there will always be negative posts and that doesn't mean the place won't be a wonderful fit for your child.