Funny - I just brought this up on our site here. The ELNs I've seen have been quite inflexible, essentially involving filling in a table for each experiment you've done. I've never had a look at how easy/intuitive it is to link or tag experiments, or do other things like colour-code experiments for importance/type or visualise groups of experiments/data. I was wondering about it since I was thinking if there were an open source version of an ELN (for e.g. organic chemistry) this would I think accelerate the number of students keeping notebooks online.