...for writing on labels - tried this last night and it seemed to work - a pen for writing on freezer labels (mine's from Lakeland). Seems rub proof and water proof on a standard plant label... so far anyway.
Ten x
I use some permanent felt tips from Wilkinson - 3 for 99p. Not too chunky and nice and cheap (I'm always losing them). CD pens are also good - often come included with a pack of blank CDs.
Unfortunately, the so called waterproof/permanent pens didn't work for me very well last year. Pencil was slightly better. So this year, using Dymo label thing (the junior version) and at less than a £10 with a couple of spare rolls of tape (there are some up for sale on eBay as well)....
All my propogator trays are numbered with 'puter printed numbers on paper. I cut them out and stick them on the base of the propogator (not the lids.) And cover/stick them on with a bit of see through medical tape. Sticks well through all the watering. The numbers/details are logged in a diary at the sowing/pricking out time. One or two pots are marked with indelible marker when I prick out into plastic pots which I put onto plastic trays.
(I use, in the main, those plastic drinking cups for vending m/c's. Cheaper even than washing out and sterilizing plant-pots.)