collaboration tool but not sharpoint

Hi Experts

i am searching for some sort of tool or way to give around 20 users possibility to work on shared directory which include MS office files

like word excel etc

we have multi shared file systems  and in one of these file shared file system we have one directory which include excel and word files

so some users want to access this off line then when they jion our network they want to sync

the problem is some time 10 users work on one file

any idea

thanks