There are lots of control panels available for people to use that are free and that you can use on Linux for your clients, however I don't recommend it. One you can use it Webmin and Usermin - Free, not the best, but pretty good since the fact it is free.
The majority of free ones are not totally the best, however the best free one that I have found is called web://cp
This is a very good classic. http://webcp.can-host.com/
I liked it a lot, never used it though for clients, but it's pretty good I find.