It surprised me to learn that many CAD users still don't understand the awesomeness of the multileader. Of those who do use multileaders, many users do not know about one of their coolest features: block options.
When you launch the multileader command dialog box, click on the Content tab. Change the multileader type from Mtext to block.
This opens up the source block ability to choose block options for all sorts of common callouts and piece mark style labels.
There are a handful of commonly used shapes that most people will find useful, but users can also apply their own blocks if they so desire. These blocks added to the multileaders share all the benefits of the already powerful multileaders.
Notes from Cadalyst tip reviewer R.K. McSwain: It should also be noted that you can use blocks with attributes on multileaders. As Scott notes, this feature opens up many possibilities for custom leader annotation.