There is a really cool utility from the selinux folks called sandbox. It's lets you run an application inside a sandbox which has limited permissions on the system. The idea being that you could run an untrusted process which shouldn't be able to cause any real damage. I dare say these days the most untrusted process is a web browser. I know Chrome uses a technology similar to this where each tab gets its own sandbox, but I don't run Chrome, so my goal is to make Firefox as safe as possible. Plus I'm a paranoid nut, so this sort of thing I find really interesting.
The sanbox program is part of the policycoreutils-python package in Fedora. It has the unique feature of being able to run an X application inside the sandbox. This is done by using a Xephyr X server. Getting this to run Firefox the way I wanted took a bit of work, but it's quite handy now that I have it working.
The biggest advantage I now have are multiple browsers running as my user. I have one browser for general browsing. This browser I never enter a password into, as I presume some of the sites I visit could be malicious in nature.
My other browser is for trusted sites, like webmail and my bank. I'm able to run any number of browsers I wish, since each runs in its own sandbox, I don't have to worry about any resource collisions. If I have a questionable site to investigate (which happens in the security world fairly often), I just run another browser, check the site then close it. The sandbox cleans up any mess left behind when I'm done.