There are a number of different interfaces to make an place experiments online, e.g. in questions https://cogsci.stackexchange.com/questions/699/software-for-online-psychological-experiments-that-dont-require-users-to-downlo or https://cogsci.stackexchange.com/questions/109/open-source-software-for-running-internet-psychological-experiments-that-collect.
We have an experimental design that we'd share online, but we've found that one feature may be quite rare - namely counterbalancing. We have 4 conditions, and we'd like participants to be placed in these conditions so that we have more or less equal amount in each by the end. This can be difficult for web studies since it should only count the participants that successfully completed the trial.
Free and open source would be best, we don't have a designated budget to pay for setting up an experiment.
I hope this question is not too general. Thanks!