Pretty sure there are no HUDs that are for free (free trials of course, but not free permanently). With this said, I hear good things about pokertracker and holdem manager - while I hear that DriveHUD is the best "cheaper" option.
I currently don't use a HUD regularly, but I did use the DriveHUD free trial a while back. It was pretty good and I really might consider purchasing it. I've been doing all right in my freerolls without a HUD, but it seems like an "almost-free" investment since even freerolls should easily pay back the cost.
Whichever tracking program is selected, I'd personally be most interested in post-game statistics and analysis for fixing my own leaks rather than mid-game advantages (although clearly a nice benefit as well).