WSO2’s cloud-enabled, 100% open source enterprise service bus (ESB)  is a standards-based messaging engine that provides the value of messaging without writing code. It provides data integration capabilities, eliminating the need to use a separate data services server for your integration processes.
In general, ESB has to offer a wide range of integration capabilities from simple message routing to complicated systems which use integrated solutions. When you find when we should use WSO2 ESB, you may find main capabilities as follows;
- Message mediation: Manipulate the message content, direction,
destination, and protocols with message flow configurations.
- Service virtualization: Wrap existing systems or services with new
- Protocol conversion: Bridge different protocols. For example, JMS
- Support for Enterprise Integration Patterns (EIP) 
- Quality of service: Apply security, throttling, and caching.
- Connecting to legacy and proprietary systems: Business adapters,
including SAP, FIX, and HL7.
- Connectors to cloud services and APIs: Salesforce, Twitter, PayPal,
and many more.
- Configuration driven: Most functionalities are driven by
configuration but not code.
- Extensible: There are extension points that can be used to
integrate with any custom protocol or proprietary system.
WSO2 ESB considered as light weight, stateless integration bus which can be a good competitor in open source environment. Please refer following links to find more interesting capabilities of WSO2 ESB.