Scribble said:
I love new optional rules.
Things that you can add into an existing game. For Instance:
...
See what I mean? Rules that probably won't make a dramatic change to the way the game is played but add a neat new element to the game...
What other new rules do you guys like?
What books are chock full of em?
Ok, you may really like Unearthed Arcana. It actually has 3 kinds of variants: character stuff (can be used alongside the core classes/races, or as a replacement), rule variations (supersede the core rule) or rule addition (add something without changing the core rules). Sounds like it's the third kind you'r looking for

and of this type UA has at least the following:
Complex Skill Checks: breaking up several skills into more checks for a single but extended task
Character Background: not exactly "stuff for PC", it's a collection of tables to suggest character choices depending on the off-adventure activities; it can be used in different ways
Combat Facing: largely expanded rules for combat manoeuvres
Summon Monster Variants: not really a rule but just a guideline/suggestion about making themed summoning lists for specific casters
Metamagic Components: about the use of expensive ingredients to replicate metamagic improvement to spells (not a general rule but specific tables)
Spontaneous Metamagic: can be used alongside normal metamagic (although it's more convenient), it's a variant take on the whole mechanics of metamagic
Item Familiars: says it all
Incantations: some spells available to everyone through complex casting
Contacts: rules about keeping contacts for PCs besides the ones obtained from roleplaying
Reputation: about how your history and deeds affect non-combat interaction
Honor: in a way an expansion to alignment, but doesn't fit in every setting
Taint: another sort of alignment expansion with in-game effects
Sanity: says it all
Some of these such as Facing and Sanity are VERY detailed and as such they are a huge addition to the game, while others such as Contacts are very easy to drop into the current ruleset.
Then of course the whole rest of the book has other variants, although in the simple form of race/class variations, new types of character crunchy bits (traits, flaws, spelltouched feats), new ways of spending levels (legendary weapons scions, bloodlines, paragon classes) which can be easily added to normal settings; other variants are real changes e.g. to how armor works, to how damage and injury work, or to how spellcasting works.
Check out my review if you wish
http://www.enworld.org/reviews/index.php?sub=yes&where=active&reviewer=Li+Shenron&product=UA