We use Ecover liquid.
Oh yeah, just a quick note that I feel is worthy of mention here....I don't have a tumble drier and was getting peed off with having crispy nappies so I thought 'what the heck' and shoved about 1/4 of a cup of comfort softener in with my nappies. Needless to say, I got through 4 pairs of trousers and 4 vests the day after. The following day was spent stripping the little sods. I can't believe I forgot to tell you girls this!!
I'm actually considering switching back to the powder though, as liquids are meant to be bad for your machine and cause more of a build up in the nappies.