Below a summary of all free items to be found in The Sims 3 Store. Note that you need certain updates/expansions to use some of these (is always mentioned). It is advised to download free items right away cause some items may be removed at some point.
With many thanks to 'Walden95' from the Sims forums for helping to find some items.
*No longer available in the Store. Will remove it by EA's request.
**Only available through the In-Game store.
***Part of a different free Christmas set.
If you're registered with EA, you will get an e-mail on your birthday with a link to a free pack. Read all about it here: Sims3Forumpost