Matty has posted his own Top 10 DTrace scripts. These are really useful and easy to use tools based on DTrace - if you don't know DTrace and want to know more about what's going on in your system you should check them. Even if you know DTrace you should probably look at them - there's always something to learn about.
Another collection of useful tools based on DTrace can be found here. And againa - these are ready to use tools for every sys admin even without understanding DTrace.