Is there a software, possibly free, that'll block a website at certain hours but not at other times?

If you're looking to keep yourself or others on your network from going to sites at different hours your best bet is to use a firewall. A Linksys router's interface, for instance, includes a section called Access Restriction where you can block by website address, including certain times of the day or days of the week. I have also heard of a new software on the market for parents to control their children's online time, but I hear reports of computer-savvy kids being able to work their way around it. If you do use a firewall already, check what its settings let you do and make sure you have set a secure password on it which your children would not be able to guess.

If u know Unix shellscripting, then u can make a script that does exactly what you've described... (presupposing your OS is Unix or any Unix-variant...)

netnanny which blocks all the porn sites frm children with password

the "isa server" software could handle your request.