[al1x]

Many of these things are covered by elance's Independent Contractor Services Agreement, found here: https://www.elance.com/p/legal/Independent-Contractor-Servic...

IANAL but supplemental agreements may not even be necessary.

[m4wk3r]

http://help.elance.com/entries/34758-sample-contract-agreeme...

elance has a sample nda and mnda available. There must be some use cases where supplemental agreements are useful.