HouseRules

Every RPG is a selection of rules that are mostly optional. And most groups not only use only a subset of those rules, but quite a few have their own house rules. Rules they invented themselves to handle situations that were either not foreseen in the original books, or were not satisfactory to the group, or - the group simply wanted to have handled differently.

There several pages for house rules:

Character Creation House Rules

Combat Skill Split

Mythos Magic

To Do This section may grow fast in future, and pages may become too long. I suggest that instead single page for each type of house rules, a list of item is created and each rule or group of interconnected rules gets it's own page. Also, I suggest a two column layout, with balanced number of sections on each column. Each section can have separate page for each sizable or separate rule and a "MiscRules" page for the small tidbits. I can do that if I get approval from lead editor.(author-RaynerApe)

Some other websites have solid House Rules worth giving a try.

TBone's GURPS diner - All your 3rd Ed. needs for realistic growing and shrinking, plus tons more!
The Fantasy Geographic Society maintains its own repository of house rules and GURPS characters.

While not so popular, many other websites exist that provide interesting House Rules:

Justin's GURPS House Rules offers a list of small house rules for the latest edition of GURPS.
Eric's Dominion provides several articles, new talents and house rules for GURPS.
Gurps Ground Zero good collection of houserules and more. Most interesting rules are the arete compendium and these thoughts on making combat have a more intense feel.