There's some really good articles on this topic here.
My personal arguments against a Christian god:
1) If the Christian god is truly omnipotent, it would mean he would know everything--past, present, and future--therefore; when he made us, he had already known that things would go to hell (no pun intended). If god is benevolent, he would not have made us just to see his creation suffer. This includes the fact that he created a world that is naturally destructive and violent (e.g. earthquakes, tsunamis, volcanoes, hurricanes). Therefore, god is not moral, contrary to the Bible, so evil and suffering contradict existence of god.
2) If God gave us free will, he would not have placed upon us the condition that we must worship him and no other god. In fact, this gives perfect justification for going to war in the name of the "right" god. In the course of history, when you realize how many wars were fought in the name of god, there is a question of why God would want millions to die for him if he wanted people to do good and right in his name.
On the more scientific side:
3) Is the Universe logically possible?
"God is thought of a being who is all-powerful, but can he make a square-circle, or a triangular-square? If he can do these things, then it would push God into the realm of that which cannot possibly exist. This is why most philosophers understand the meaning of "omnipotence" as the ability to do that which is logically possible, and any questions as to whether He can do things that are logically impossible are thought to be meaningless.
This is why I want to ask, is it logically impossible for the universe to come from nothing? If it is logically impossible, then god cannot do it. However, if it is logically possible, then it could be reasonably said that the universe came on its' own. Then to postulate the idea of God might be explanatorily superfluous." -By Samuel Owens
4) God doesn't need a cause because it has existed forever.
In order to avoid the problem of the Universe being an uncaused cause some say that god has always existed and has therefore never been created: It therefore needs no cause. But I find this is an argument that can be used in conjunction with big bang theory to prove, again, that god is not likely to exist as a first cause. Because according to some Big Bang theories there has been an infinite number of cycles of Big Bang / Big Crunch (where the Universe ends in a big black hole after contracting) and that the Universe has existed forever. If it is possible for something to exist forever and not need a cause then it is likely to be the Universe, not God, and once again we can theorize that this is likely to be true because there would be no reason for god if it was true that something could exist with no cause.
Big Bang models describe the creation of all the dimensions of space including time itself, so that to ask "what happened before the big bang?" is meaningless in the same way as asking "what was god doing before it created time?". -By Vexen Crabtree
Personally, I am agnostic, despite strong arguments on either side. The word “agnostic” essential means “without knowledge.” Agnosticism is, I suppose, a more intellectually honest form of atheism. Atheism claims that God does not exist – an improvable position. Agnosticism argues that God’s existence cannot be proven or unproven – that it is impossible to know whether God exists, which is inarguably true.
Okay, I'm finished.