I don't have an active subscription so there won't be updates to these icon packs for awhile. Most of the icons are still skinned but the new icons Blizzard occasionally adds won't be. Sorry! I'm opening the packs up to patching so if you want to make a patch with the new icons feel free to do so -- I'd appreciate the help.

About
sil_ICON Vanilla Icon Replacement Packs are replacements for Blizzard's 64x64 pixel icon blp files. This particular pack features a light gray simple square border.

Install
Extract the zip and place the ICONS folder into your Interface folder.

Contents
Icon replacement packs like this one should not affect your game's performance. Our icons will load instead of Blizzard's default icons not in addition to. Regardless, you may not need all of your icons skinned and are pressed for room on your hard drive. In such a case you may want to delete some of the BLPs you don't want. Here's a nifty guide to the contents of our ICONS folder:

• BLPs beginning with Achievement_ are used in the Achievement window
• BLPs beginning with INV_ are inventory items seen in your bags or character sheet
• BLPs beginning with Item_ are other loot icons
• BLPs beginning with Ability_Mount_ are mount icons (a few are used for spells)

Those alone account for the majority of the BLPs. If you're looking for specific icons there's an Icon Filename Directory on WoWWiki you can reference.

Requests
I'm accepting requests for more vanilla icon replacement packs. Leave them in the comments. Please understand though that batch icon skinning was hard to figure out, wasn't necessarily easy to do and took quite a bit of time.

Credits
The skin I used in the process of making these icons I pulled out of Zork's rActionButtonStyler - simplebutton skin

Links
Other Skins:

sil_ICON:SimpleSquareDK
sil_ICON:Thin
sil_ICON:VeryThin
Clean Icons
Clean Icons - Square
AI-Art Icons

Programs Used:

Gimp 2.6
Blpc
XnView
ImageMagick

Reference:

Wowpedia:WoW Icons