Wargame Rules

Included below are links to a bunch of free wargame rules sites, including both pre-established wargame rules and free wargame rules that people just made for fun.
For those of you who do not feel like roping themselves into a pre-established wargame system, there are still tons of free wargame rules out there for you to try out without launching onto a spending spree of a few hundred bucks as you try to buy all the rulebooks, armies, and models you will need to start a new system.
Maybe some of the free rules below will strike your fancy. Some of the rules below can be found at freewargamesrules, but I have included the rules individually for easy access.
I must also admit that I have not personally tried out all of these rules, but I have bookmarked them with the intention to someday getting around to playing them.
Also, I strongly encourage anyone with the inclination to try making your own rules. It's not as hard as it sounds. Just scan some similar wargames rules and mix and match to customize a set of rules that is perfect for you. If you have no idea where to start, check out my page on making your own wargame.

freewargamesruleswikia
This site has led me to innumerable great wargame rules. I also usually modify these rules to fit my own needs. Since they are free and not copyrighted, and I am not trying to distribute or sell my modified versions, I can see nothing illegal in this.

Free War Games Rules
Despite having a very similar site name (except for capitalization and spacing) this is a different site from freewargamesrules.

Free Rules THW 

These rules are complete and completely free rulebooks by Two Hour Wargames. One is Chain Reaction 3.1 (modern skirmish), the other is Swordplay (ancient, medieval, and fantasy skirmish). They come in the form of pdfs and you can download them for $0.00 at the time of writing. Note that these are skirmish style wargames, for small forces of miniatures. Each miniature = 1 soldier.
I have downloaded both rulebooks, just cuz they're free and they're there.

The Perfect Captain
This is another great site for free wargame rules. They have rules for Ancient Greece, the Wars of the Roses, the Crusades, and more. 
DBx:
Play DBA! This is a great page for someone new to gaming, and, especially, DBA, who wants to start playing quickly and without going through a ton of research.

Fanaticus  Guide to all things DBA

The Unofficial Guide to DBA self-explanatory. This is not a substitute for the official rule-book, because it does not include army lists, but it is a great way to get started

DBA 2.2 Rulebook link to Amazon.com

DBSA The original rules for DBA, created in 1989. These are quicker and far less complicated, but also less well developed.

Wadbag DBA v2.2+ These are free, online rules for an expanded version of DBA 2.2. All things pertaining to v2.2+ are included in the link.

DBA 2.0 War of the Ring An imagnative adaptation of DBA for fantasy armies - more specifically, the Lord of the Rings. This page contains only army lists; normal DBA rules are used for game play.

DBM: Middle-Earth Army Lists  This page contains well-researched and thoughtful army lists and army notes for LotR DBM games. I include this link under "DBA" because DBA army lists are also included for each army at the bottom of its page.This page is geared more toward DBM than DBA, but it does have a lot more army lists than David Kuijt's page "DBA 2.0 War of the Ring."

Also see my post on additional DBA fantasy army lists.

 DBN Napoleonic Wargaming the official site for the Napoleonic version of DBA

World War II wargame rules:
Focused Firepower
Combat Team!

Modern wargame rules:
Counterattack

Ancient wargame rules:
An Age of Sword and Arrow
Age of Heroes

And here are some really cool gladiator combat rules. I have not had time to look at them in depth, but maybe you will:
Are You Not Entertained?
Munera Sine Missione

Fantasy wargame rules:
Both of these rules require a massive collection of multi-sided dice, but if you got the dice, or a way of obtaining them, good for you.
Fast Play Fantasy Rules