There is a whole lot of themes available for the iPhone and iPad on the Cydia repository. Unfortunately most of them are actually darn ugly - like really ugly. I don't know what people are thinking when they pay for these god awful themes.
As always though, there is one or two which deserve a lot of attention due to the amount of time, effort and craftsmanship put into them. One of my all time favourite Cydia themes is Jaku. It's easily one of the most beautiful themes available for iPhone and more than deserves the $2.99 cost for it.
However if you're looking for a free theme which has pixel perfect icon designs, Circles is for you. Jackie Tran (only if his surname was Chan) from Prague, Czech Republic, has released what I personally think is the best free theme available for the iPhone and iPad.
It's perfectly made for both the iPhone and iPad retina displays with the iPhone icons in the size of 114x114 and the iPad icons in the size of 144x144. The amount of detail put into each icon is really incredible - even the shadows behind the icons are adjusted to make it look like they're floating on the screen.
I've never been a fan of iOS' default over-glossy icon design and have always favoured matt-like icons - Circles provides this matt look. All default iOS app icons have been changed, including some major apps from the App Store, such as Twitter, Instagram, Facebook and many more. Icons for apps which haven't been customised still get the circles look by cutting all icons literally into circles. It's very hit and miss though because obviously not all app icons will look great cut into circles.
If you want to be creative, Jackie even provides the PSD for you to create your own icons so you can get them pixel perfect.
Seriously, if you're looking for the perfect free icon theme for your retina iPhone or iPad - this is it.
UPDATE July 2013: Since writing this post back in Septermber 2012, Jackie has now attached a price to his theme. But in the process he has also added more icons and given the ability to create your own.