Tigi make a good conditioner but dont leave any in overnight as you'll itch like mad and snap your hair. Leave for 30mins at most, towel wrapped to keep head warm.
I use Pantene Time Renewal nightime leave conditioner. I also use a heat protection spray which I would recommend. GHD do a good one which I have just run out of so I got Charles Worthington.
Loreal - the pink range for detangling and reviving dull hair, don't know if they do a leave in conditioner - but it's fabby. Really makes hair shiny, soft and swishy.
Redken do a brilliant leave-in conditioner - it comes in a large blue bottle. Just put a few suirts thourgh your hair after washing, and dry as normal. It smells yummy, and leaves your hair really shiny and soft. Here is a link to it - I know it's a US site, but you can buy it here too.