It is of course Cardcaptor Sakura by CLAMP.
In the first episode 10 year old Kinomoto Sakura is drawn to a book in her father's office that contains a deck of magical cards, and accidentally scatters all but one of the cards to the four winds. Oops. The guardian of the cards then makes it Sakura's responsibility to collect the cards to avert an unspoken disaster, and grants Sakura a limited set of powers for this purpose. Basically a limited spell to bind card back into its proper form, a wand/staff/baton, and use of the one card (Windy) that Sakura managed to hold on to.
It is the limitations that help lift CCS far above the average magical girl show. The limitations mean that Sakura has to work, and get downright creative sometimes, to solve the problems presented by each card. This makes for a much cleaner message of empowerment and avoids the altermative message of external granting of power that can be present in some magical girl shows (something that Steveg and I discussed at the Anime Archetypes panel at SwanCon this year).
Another aspect that I'm fond of is the avoidance of tacky stock footage for transformation sequences. Sakura's key transforms into a wand/staff, she doesn't, and neither do her costumes. In fact the costumes that she does wear are a running gag but they get made the old fashoned way by one of the supporting characters.
Which leads me into main reason why I adore CCS so much: the characters. They are simply a delight to watch, particularly with the gorgeous character designs (this is CLAMP we're talking about so duh!). Particularly worth noting is the relationship between Sakura and her big brother Toya which is just spot on: the superficial bullying/teasing that covers a deeply protective love for a younger sibling. Reconciliations within/between families also comes up and is handled well.
In short CCS has been one of my favourites for over 10 years now and I thoroughly recommend it to anyone looking for a show that is sweet without being saccharin and filled with a (mostly) innocent sense of wonder.
Although you should fear Tomoyo-chan. :)
NB: Suggestions are still being taken for the topic list over here.
Day 1 - Very first anime: Robotech, Gall Force, Vampire Princess Miyu
Day 2 - Favorite TV series - Card Captor Sakura
Day 3 - Your anime crush
Day 4 - Favourite Film
Day 5 - Anime character you feel you are most like (or wish you were)
Day 6 - Most annoying anime character
Day 7 - Character you’d most like to slap
Day 8 - Most epic scene ever
Day 9 - Saddest anime scene
Day 10 - Favorite slice of life anime
Day 11 - Favorite mech series
Day 12 - Best instalment of a franchise you don’t like
Day 13 - Worst instalment of a franchise you do like.
Day 14 - Current (or most recent) anime wallpaper
Day 15 - A reality check for a genre you like
Day 16 - An anime classic that simply doesn’t work for you.
Day 17 - Best Background Music
Day 18 - Best Five-team (?)
Day 19 - Change you’d like to make to any one anime
Day 20 - Most ambiguous anime character
Day 21 – Favourite OAV Series
Day 22 - Animation studio you'd most like to slap.
Day 23 - Anime you think had the best, or most intriguing art
Day 24 - Favorite anime hero or heroine
Day 25 - Best anime villain
Day 26 - Your favorite harem anime
Day 27 - Favorite anime opening theme song
Day 28 - Favorite pokemon (?)
Day 29 - Best Anime Music Video
Day 30 - Favorite anime ending theme