Heterogeneous computing is rapidly gaining increased attention due to the promise it holds in overcoming power and performance walls in traditional computing systems. With its focus on customized processing nodes dedicated to the different tasks in an application, it is hoped that these walls will be overcome. Therefore, CPU-FPGA co-architectures are also gaining ground in application areas like recognition, mining, search, datacenter etc. However, research in CPU-FPGA co-architecture is constrained by the available synthesis and simulation tools which do not provide an integrated system level simulation and architectural exploration environment. This becomes critical when we incorporate novel memory hierarchies, multi-processor chip architectures, hardware level cache coherence etc. In this paper, we describe our open source and integrated system level simulator and architecture exploration tool called HeteroSim. It supports x86 based multi-core processor combined with a FPGA via bus-based architecture. It allows integrated system level simulation and returns performance metrics to understand application performance with respect to the simulated architectural configuration.