I guess you mean compost for seed trays and module sowing/pricking out?
I have a few things to say on that and shall post them in April 2012 in a couple of days. Meanwhile, let’s just say a basic seed compost for sowing most seeds, such as John Innes no.1, and for larger seeds or pricking out, most multipurpose do well. If you can access it, Gardening Which did a useful trial recently and found a few shockers, but for example B&Q’s grew well and is cheap.