We propose a software platform for applications based on socially intelligent agents (SIAs). SIAs are software agents which manifest social intelligence. Although there are many types of social intelligence, we focus on two types, social intelligence for providing emotional support and social intelligence for providing cognitive support. By applying these types of social intelligence, our SIAs are able to simulate human social behavior. We demonstrate how we created the SIAs and describe the conceptual architecture of our software platform. We also show the mechanism for generating the agents' social behavior, which makes our software platform unique. According to the media equation theory, we expect SIAs to benefit users. To examine the effects of SIAs, we designed and implemented two applications. We provide explanation about how SIAs function in these applications and discuss further implications of social intelligence.