I was wondering what kind of program if any do you use to Bill your clients...

I had some problems with some late clients this month

I would check out the following programs:

www.modernbill.com
www.ubersmith.com

I've used both programs and each has its own advantages and disadvantages.

Some other hosting companies use two other programs by:

www.optigold.com
www.mrtec.com

I've played around with the trial of MrTec's program but didn't like it too much. It does do what it's supposed to though. Optigold, installed it but never touched it.

For something free, you might want to check out CMBS from Voxel, makers of ubersmith. I've used it a few times but required too much customizing.

http://www.voxel.net/projects/cbms

Hope that helps!