Well... let's talk about what "safe" means in the context of CC.
Safe means that the content in question:
* Isn't directly defective.
* Doesn't contain any really harmful code.
* Has ID numbers that don't conflict with already-installed content.
* Doesn't mess your game up in other, unforeseen ways.
The first two are fairly easy to lick. Get Custard, get Sims Dashboard (Google them, they're all over the place last I checked). They work on the PC and are easy to learn to use. Then don't install a blessed thing on your game without it going through those two programs (Custard if it's a sims3pack, Dashboard if it's a package file).
The second two are what will really mess you up. Even if content passes the first two tests, it might fail the second two because--and this is important to understand here--almost all CC is written by amateurs who may or may not understand what they're doing and how it interacts with EA's content. It's crazy hard to catch conflicts (Dashboard will run the check, but it's not always easy to match up the actual installed file with the conflicted item's ID), and it can be almost impossible to tell if the content will otherwise mess you up till you put it into game and then realize everybody's naked. Or the game is crashing to desktop. Or your Store won't download to your computer. Or nobody's getting married. Or your Launcher decides it hates everybody and won't load. Or everybody's bald. Or, as someone in this very thread discovered, that awesome foreign site gave you a horrible virus along with the CC you downloaded. CC can have really unpredictable effects on your game, and your results might vary from mine because we have different stuff installed on our respective games. Every one of the sites mentioned here as "safe" has given me problems at one time or another.
That said, I am the queen of CC. I love how it adds to my enjoyment of the game. But all that is why nobody can really say that any site's offerings are "safe"... because there isn't any such thing as "safe" in the world of custom content. You need to be using those two programs if you want to use CC, and even then, don't install something just to be installing it. Install it because you genuinely want to use it and know you'll enjoy having it, and install stuff verrrrry slowly, like 1-3 things at a time. After a batch of installs, check the Store, and load the game to actually get a look at the new additions--put them into a corner of your Sim's lot, and run the game with them there--work, go on vacation somewhere, change your Sim's appearance a couple times, etc. If something doesn't work, you now know exactly where the issue is. Paranoid? Yes. But it's helped me run a good clean game for a while now.
