I was looking in the Plugin Requests sub-forum on the Bukkit forum, and I found a request that seemed like something I could do easily. It was for a plugin called Bow Powers, which basically made it so there were different kinds of bows. Things like exploding arrows, lightning arrows, arrows of slowness, web arrows, and poison arrows. It sounded cool, so I started. Basically, you left click with your bow to change what kind of arrow you want. It wasn't too hard to create, and I even made arrow trails for each one! With the exception of the exploding arrow, all the arrow trails are just the particles of certain blocks getting destroyed. The poison arrow I especially like, because I used the breaking crops particle which looked a lot like dripping green poison. Basically, here are the different arrow trails:
Lightning: Quartz Block Breaking (white)
Poison: Ungrown crops breaking (green)
Slowness: Soul sand Breaking
Web: Web breaking
Grappling: Gravel Breaking
Explosive: A bunch of mini explosions as a trail (no block/entity damage)
It really looks cool when they fire. The only real bug that I found is the grappling one. You basically 'hold onto' the arrow as it goes, but sometimes it hits you. I made it so you don't take damage, but it still destroys the arrow. This only happens sometimes. I will release the plugin for download when I am done with it (permissions, small bug fixes, etc)!