Chess.com would prefer that you step up to a diamond membership which would allow you to do unlimited TT puzzles, do the tactics courses on Chess Mentor and watch related videos (IM Danny Rensch just finished a killer series reviewing all the tactical motifs). That's a lot for $100 a year. You said free though.... Staff frowns on recommending rival chess sites, so I won't, but I bet a little googling around would yield a free tactical trainer site.
What is the best way to learn tactics?

Maybe joining a chess club. I've been to a few chess club meetings at my school and have learned some new tatics that have helped me.

Thanks Dargone i have looked into the book and i am thinking about buying it. I also found a cool little website literally chesstactics.org, that neatly explains a lot about tactics (its not a joinable website). It shows how to effectively fork, skewer, xray, etc. I would recommend it to anyone looking to brush up on their skills. Thanks for the information guys

Joining a chess club may help, even in a tournament. You will learn many things when you look at master's games or even your own games. :)

Chess.com staff guy (forgot his name) mentioned a system a year or so ago.
Forgot the specifics, but the idea was that the goal of tactical exercises for tactical skill is to learn new patterns. So set a time limit (like 5 minutes) and if you haven't solved it look at the solution. After failing 5 or 10 stop and go over those failed puzzles until you can do them easily / blindfolded / whatever. Learn a few new patterns each session and you're on your way.
I used to do them (without knowing it) for calculation/visualization practice. I gave myself no time limit, and would not move the pieces or write anything until I was satisfied with my analysis. Then I'd write all the variations I saw down, as far out as I thought they might require. After 10 or so puzzles I'd check all my answers at once. After about a month I found this really helped my OTB organization / analysis ability. I found myself rarely needing to re-check a variation during play. (I'm rusty now so I'm back to fumbling though multiple lines at once, re-checking as I go, with a final check at the end heh).
The title says it all. I do have multiple chess books, and they are quite useful. Unfortunately, books cost money, so i was wondering what is the best free way to learn tactics otherwise. Is there a website dedicated to it? or is the best way to watch master games? or is it just to play? or perhaps a chess teacher? I would like to get better at tactics, as when I analyze games afterward with an engine I frequently notice that I often make the "obvious" move, rather than the more destructive, subtle attack. In summary, I would like to know the best and most productive way to improve tactics that is free or extremely cost effective.
PS: I have no need to study openings and endgames as of right now, most games my level end far before the game enters endgame theory. I know the most basic openings (kings pawn, italian, sicilian, kings indian, french), but see no practical need to learn the tedious openings
Thanks to everyone!