Addins are a great way to addd functions and tools to Inventor. Check out the newsgroup postings.
If you want to try some on your own there are a few add-ins that are delivered with Inventor. Go to C:\Program Files\Autodesk\Inventor 11\SDK\Tools\Users. Here you will find a few add-ins that offer some nice "bonus" tools.
Sean Doson also has a great site to check out here: