The blocker I know best is K9. It occasionally clashes with anti-virus stuff, but only when said anti-virus has some advanced features turned on that overlap with what K9 is doing. In my case it was only when I was in the "free trial" of their premium version.
For accountability software, the one I've heard most about is Covenant Eyes. I never used it so I can't personally vouch, but I'll throw it out there as a suggestion.
A good free blocker is Pluckeye. I just responded to another post giving a little detail on the program. Don't know any good accountability programs though.