There is no secret that I am a big Solaris fan. For me as a developer ZFS and DTrace changed my life, improoving my productivity significantly. With ZFS and its snapshoting/cloning capabilities, I have no fear in doing experiments and I can always go back in time and recover in case something goes wrong. On the other hand DTrace with D-Light allows you to look inside your running application.
If you want to learn more there is an excellent tutorial that describes how to use DLight to profile your applications: