There are probably lots of ways to use the Illustrator Brush Engine to make life easier, I just wanted to point out 10. I would like to credit Veerle and her post about Brushes which indirectly prompted me to think about this.

1. The swirly curls from Veerle, could be accomplished with a single curl and the application of a brush like this. Note the simple variations that yield impressive results.
2. I love the Art Brush Setting, but what I love even more is the Pattern setting. Use it too make a Celtic/Greek Key pattern like this. Not that simple, but super powerful. If you want the corner and end pieces, you need to add the object to the swatch palette.
   
3. Another brush setting that is amazingly versatile is the Scatter, aside from the leaves and twigs that are pervasive on the internet, there is also the Kirby Crackle, which lends itself to this brush quite nicely. A Tutorial on the how and why of the KC is here at Pencil Jack. This is the most deceptivly simple of the brushes I am showing; it is one black circle with scatter settings that are tuned to produce random-esque dots. I set size to my pen pressure, if you aren’t using a tablet, you should be.
4. Not to toot my own horn, but my Cables and Cogs is a reasonable implementation of the pattern brush that I made for the Steam Punk community, and has since been ripped off by others, but the original is still the best.
5. In Cartography, there are certain brushes that are indispensable, but be warned… many of the roads can be made by just layering the strokes and using the Dashes and dots to give the appearance of blacktop… but with pattern brushes.