Providing contact to website users

How to provide the possiblity to contact a certain entity is difficult for many companies and organizations. Especially for small NGOs, it is not possible to have a big mailing infrastructure and to maintain server-side spam filtering. Therefore, email addresses should not be published as plaintext. Nevertheless, the website of the NGO should somehow offer the possibility to contact the NGO. It is important that the technology used is widespread and cheap, since NGOs do not have a large amount of money to spend on specialized technology. If possible, open source software should be used. As the existing infrastructure (apache server, msql server) is offered for free, it should be used to solve this problem. The only important thing is that spamming should not be possible and the tool should guarantee that a human being is trying to make contact, not an (evil) software program.