cpulimit for FreeBSD
Using the /etc/login.conf file, you can set a limit on max CPU time a process is allowed to consume, but it's not currently possible to limit the percentage of the CPU an individual process uses. (Reference here)
You might be able to accomplish a similar effect by running the process in a jail and limiting the resources for the jail
There is also a new utility, cpuset , which you can use to limit certain processes to certain cpu's.