What a scary situation! I haven't read all of the responses, so maybe someone has already suggested it, but I know they have paternity tests at your local drug store. I think they cost between $20-30. If you are in contact with the other guy you can ask him to take it (I think it's a cheek swab) or you can come clean with your husband.
I am pregnant, and things were pretty rocky a few months before I became pregnant. I had just moved in with my on/off bf of 2 years when 2 weeks later, BAM! Prego! And strangely, it has been the best thing that could have ever happened. Now, babies aren't meant to fix relationships, but I personally think you should wait to see what happens and get the paternity test done. Even if you have to swab your hubby in the middle of the night or make up some excuse about it detecting the babies gender or something, or maybe the swab tells you of your exact lineage. I dunno. It's really up to you to decide! I personally could never get an abortion, but that is me, not you.

You do what you feel is best.