Wincustomize daily easiest/quickest to navigate.
Aqua-soft.org daily which is a forum and more emulation oriented
Deviantart daily but, it's becoming a big time waster.
Hollywoodgui daily

Occasionally:
MaxStyles
Skinnalicious
Customize.org
Skinnables



And I thought it was just me!

SKinartistry
Skinnalicious
WC
DA

WinCustomize
DeviantArt
Skinbase
Winmodify

Never heard it said better!   

MaxStyles
Skinnalicious
Customize.org

same, but 99% here


  

Almost exclusively WinCustomize but I like the widescreen wallpapers on Interfacelift so I visit there occasionaly too.
I wish WinCustomize would add a sub-gallery for w.s. walls.  

deviantART
WinCustomize
SkinBase

...and I visit regularly:

Skinnalicious
CrystalXP
InterfaceLIFT
Yahoo! Widgets

I do occasionally miss the days of Customize.org,Razorart,desktopmod,skinz.org,Sonique.net,etc. Those were the some of the founding sites that really helped launch desktop customization, From which came names still hold a special place among the customization community, Dangeruss,Thredz,Doodle,Lonerunnr,Floach,Wreklesseric,to name a few.


Skinz.org, Customize.org and Razorart were my main pages, til upheavals ensued and the sites either died or became places I didn't want to frequent.

Wincustomize has grown to become the main staple of the desktop customization community, and my home page for skinning. The first site since LiteStep (or SimpleStep as it was called back then) that I actually felt compelled to become an active member.

Deviantart,I use for finding off site stuff.

Artists home sites like Boxxis,Pixtudio, etc.for releases not upped here.

LS.net just for old times sake...

I also go to the apps main page for the occasional skin not upped here. Example: winamp.com

There is also a few sites I frequent just to say hi to pals and offer some support.
Skinartisty,Shaunna's Selections,etc..

Sigh....if only we could work 'better' with Devart....

Usually the only time I'm offsite...at the other skin sites is when some 'issue' arises... eg. rips, etc.

Otherwise HG_E covered it well...

MaxStyles
Skinning.net
Deviantart
Skinartisty
CrystalXP (was the first i found)
Skinbase

i visit them once or twice a month. If not everyday i'm here

Well....I pretty much live a sheltered life I guess when it comes to skinning or customization. I spend 95% of my time here...but go off site for wallpapers and the occasional odd icon.

InterfaceLIFT is great for cool looking wallpapers, but it's about the only skinning site I patronize besides here . . .

WC
DA
Winmatrix
CrystalXP
Fasticon
Iconshock

and many, many, many more.................

I pretty Much Only come here anymore, I used to go to other sites, now I can't even remember what they were (and no that's not sarcasm I really can't remember...lol) I love the wincustomize community. It's a great website not just skin-wise but community-wise as well.

I spend most of my skinning time on Wincustomize and try to check out others sites   once in a while like, Skinartistry,Skinnalicious, DA, and Tootsie Theme Shop for Walls. That about it...  

Let me take the question a little further (in a different direction but still tangental to Zoomba's intent) . .
What gaps in WC are the other sites filling?  (Widescreen walls seems to be one thing)
What other sites should WC build a better relationship with so that everyone benefits?

Being as big as WC, if a closer affiliation were formed . . there might be some concern of a slow take-over of contnet.  Dunno . . just wondering.