Apple's iTunes US store has several apps that are now free to celebrate their fifth anniversary:

How to Cook Everything, regularly $9.99
Tiny Wings, regularly $0.99
Day One, regularly $4.99
Superbrothers: Sword & Sworcery EP, regularly $4.99
Infinity Blade II, regularly $6.99
Where's My Water?, regularly $0.99
Barefoot World Atlas, regularly $4.99
BADLAND, regularly $3.99
Over, regularly $1.99
Tiny Wings HD, regularly $2.99
Traktor DJ, regularly $19.99
Traktor DJ for iPhone, regularly $4.99

If you are in another country or viewing this on a different day, it might be worth opening iTunes and seeing if there is an anniversary sale in your local store.