I want to get a track planning software to help design my new layout. I dont really want to buy a software because I'd use it so little so I was wondering what you guys think is the best free software? Also i'd like to use Atlas O track.
It sort of depends what track you're using. The Atlas product mentioned above works well with Atlas track and the one that comes "free" as a member of the MTH Railroader club works great for MTH products and RealTrax. I am not familiar with the Atlas product but the MTH one is a dumbed down version of RR-Track which is a pretty good product which you can use free or pay to upgrade for additional track libraries and capabilities.
There are other packages that are free and/or open source but the ones I know about are primarily aimed at HO/N scales and their needs. O-scale sectional track libraries may or may not exist.
You may use the software more than you think. I find it handy to create lists of track for a given layout so I know if I can build it or what pieces I need to be able to do so. For me, the LHS is almost a 90 minute drive (each way) so I am not anxious to find myself short just one piece of track when I go to build.
Another time that I find the full version of RR-Track handy is when someone posts (or sends me) a track plan. Being able to look at it in the program vs. just a bitmap of it is very helpful.
I do understand tight budgets but this is an item you might consider shelling out the money for.