For "nearly free" hp, I've consistently seen good old fashioned tuning make significant improvements in hp and overall performance, often 20-30hp from a well running but under performing engine. I used to regularly tune the 302/2V in my Mach every weekend. This proved itself every time as I was able to get 20mpg average and run nose to nose with Fox 5.0s... all with stock single exhaust, FMX tranny, and 3.0 gears.
What to do? Start with good tune up, including new cap, rotor, wires, and plugs. Then correctly set up your distributor to provide the "right" advance curve. One of the big magazines ran an article on doing this in the last year. Next, set the timing to provide as much initial advance as possible without causing detonation. Then tune the carb. There isn't much you can do with a stock Autolite, but if you've got a Holley, get a tuning book for it. Change the jets, power valve, accelerator pump, etc. Read the plugs to check if its rich or lean.
For not so free, but generally inexpensive, the stuff NorCal suggested is good, especially an open element quality aftermarket air cleaner and K&N filter & carb spacer. Also, if you can rig up a cold air induction setup, that can also offer a bit of power.
Then for not-too-outrageously expensive mods, headers and a good dual exhaust can be the best source of hp, usually between 20-40hp over a stock manifold single exhaust. This actually can be the most cost effective, because it can easily be done for total outlay of less than $400. $10-$20 per horsepower ain't bad.
