For which of the WSO2 products are looking for an alternative? WSO2 has a lot of products (ESB, Business Process Server, Governance Registry, Identity Server, Business Rules Server, etc.). Depending on what product / functional requirements you're asking for, there may be a lot of solutions.
Some open source ESBs are:
- FUSE (based on Apache Service Mix)
- OpenESB / Glassfish ESB (Sun -> Oracle -> Independent)
- Mule
- Petals
- UltraESB
- Membrane ESB (no really ESB, but may satify your needs)
...
Some non-open source:
- Oracle ESB
- Bea AquaLogic
- Microsoft BizTalk Server
...
Some Business Process Servers:
- Intalio BPM Server
- Adeptia
- BizAgi
- Saavion
- ActiveVOS
- Activiti
...
