If you mean one that will print them then
http://www.undercoverxp.tk/ is the one i use. I think you can load a normal cover in and select to scale it to slimline too. Free with "nag screen" for donations.
Not sure of one to actually design them - I use photoshop to create and then print in undercoverxp.
Steve
Edited 1 time(s). Last edit at 11/20/2012 10:53AM by seventhsea.