I'm currently working on my flexibility like mad. I'm hoping to get front splits and my feet to my head by the end of the year. I'm also going to see where I can get with box splits - I have little hip flexibility and it sucks.
I'm adding a list of resources I use as it may help someone.
How to Stretch - this was one of the first sites that I used and it now has helpful videos
A page on splits stretches
A wealth of stretches and useful advice. It looks long and is in quite tiny writing, but the comments the author has on the stretches make me laugh.
Back stretches from Yoga.
I've not looked at this one yet, but it comes recommended. There seems to be loads of information.
I have my own stretching record that I am maintaining. There's only two sets in there, but hopefully I will be adding to that tomorrow.