When starting a project, we generally have an idea of where it is going to go. A super cool character doing some super cool thing in a super cool setting (or insert other super cool idea here). That's the plan.
Unfortunately, plans get messed up for a variety of reasons. Time and skill levels are the two most common problems that mess up our beautiful plan. Or maybe once started, we decide that we really only want to focus our energy on the main character or object.
All the added/extra super cool parts would take too long to model and texture.
So now whatí Well we could just call it good at that point and render it out and share it with our friends. Or we could browse through the ever growing collection of Add-ons and see what we could quickly add to our scene or character to take it up a notch.

add-ons save the day-1.jpg

Add-ons provide added functionality to Blender and in this case can save quite a bit of modeling time. You will still have to texture, but that can be as simple or complex as you want it to be.
Blender comes preloaded with a variety of useful Add-ons. For our purposes, the Add-ons listed under Add Curve, Add Mesh and Object in the User Preferences>Add-ons dialog are the ones we are looking for. So what can we find in there?

IvyGen: will add generated ivy to a mesh Object starting at the 3D cursor. This is a great way to quickly add some plant life to a scene.
Sapling: uses Curves to form a wide variety of trees.
ANT Landscape: adds a terrain primitive that can be adjusted to create an endless array of landscapes for your scene or character.
Bolt Factory: quickly add bolts and nuts.
This Add-on alos provides a number of options to change the look of the bolts and nuts.

Object :
Add Chain: adds a chain object with a curve for quik creation.
Cloud Generator: lets you create volumetric clouds in a variety of shapes.
Fracture Tools: breaks apart an Object, great for bombs and explosions.
Grease Scatter Objects: allows you to use the grease pencil to draw where you would like groups of Objects to be scattered around the scene. Think leaves and debris.
Quite a lot can be accomplished with these Add-ons and these are just the ones that are included in Blender. There are many more that are being written and released in the community. The BlenderWiki has a large list of Add-ons that are available to
play with.
So browse through the lists and keep an eye out on the various forums for announcements of cool new Add-ons.
They can save you a lot of modeling time, allowing you to focus on the main part of your project.