How does Camel compare to Synapse?
We are Camel developers so take what you read here with a pinch of salt. If you want to read a less biased comparison try reading this review which has a slight Synapse bias since the author mostly uses Synapse 
However we think the main differences are:
- the Camel community is way more active according to the nabble statistics (Synapse is inside the Apache Web Services bar) or by comparing Camel and Synapse on markmail.
- Camel is the default routing engine included in Apache ActiveMQ for Message Orientated middleware with EIP and Apache ServiceMix the ESB based around OSGi and JBI at Apache - both of which are very popular too
- Camel is designed from the ground up around Enterprise Integration Patterns - having a EIP pattern language implemented in Java, Spring XML, Scala and Groovy as well as EIP Visualisations
- Camel is designed to work with pretty much all kinds of transport as well as working with any Data Format. When we first looked at Synapse it was based around Axis 2 and WS-* though apparently thats no longer the case.