While familit do have a browser / content manager add-in, this is a paid product (if you are looking for free family browser, check out tools4revit).  However, the family content itself at familit is free:
At the moment familit.com offers 18.000 free families in English, German and French language.

http://lumion3d.com/

You don’t even need to register.  Just go to the following link and start downloading:
familit – Revit Family Library

EDIT Current category guide post:
https://wrw.is/2013/09/new-crowdsourced-and-open-source.html

In Revit, some families have Cut Dominance, some are Cuttable, and some respect various View Range rules in various ways.

These behaviours may further be affected by the Family Template that they were created from, whether they are Hosted or non-Hosted, whether they are Shared or not Shared.

Wouldn’t it be awesome if there was a document that went through each Family Category and described its unique properties!  I have asked Autodesk for this before, but to no avail (SR# 1-9298685321 – The help file does not describe the unique properties of each Category).

Andy Milburn shares my feeling on this matter:
… I wish I know where to find a clear explanation of all these rules.
at
Shades of Grey: CUT ! CUT ! CUT !

What about the Families Guide?

I may not have all the answers, but I have posted many times about the differences between certain Family Categories.  See some links below:

Cuttable vs Non-cuttable Family Category Properties

View Ranges (and TOLERANCES) Explained *REPOST*
Things like –
Walls shorter than 6 feet (approximately 1.83 meters) are not cut, even if they intersect the cut plane, and
There are a few categories for which an element located above the cut plane but partially below the top clip is shown in plan. These categories include windows, casework, and generic model. These objects are shown as viewed from above.

Project Family Category Type Instance
which Categories do not have Types, or which Category has Types but no Instances?

Making unhosted components like unhosted Doors and Windows

Create a Component Family with Category set to Walls (or other system family category)
Yes, it is possible.

Plan regions have no effect on Topography – workarounds

buildz: Wall Trimming Method
Cut Dominance in action

In ye olde AutoCAD days, there were a plethora of ways to mess with your fellow users.  The sheer amount of obscure yet powerful system variables gave endless opportunities for frivolity.

In Revit, not so much.  The system works.  It seems Revit does not want you to prank your coworkers, right?  Here is a little story …

We were having trouble finding a particular Revit link, or more specifically, the walls in a particular Revit link.  So we drew a roof, and the view looked like this:

Ok, so the answer seems easy.  Walls are overriden, right?  Let’s have a look in the View Template:

According to the Visibility / Graphics of Model in the View Template, everything seems cool.  Walls aren’t overridden.  All of the cells look the same, in their default state.

Or are they?

Let’s select the Walls row:

A tiny, almost invisible sliver of white appears under the Project Lines column, while all of the other columns show Override (indicating they are not overridden).

So what exactly is happening?  Let’s click on the Projection Lines override for Walls:

We get very little feedback from Revit as to what is happening until we select the actual row.  Why?  Because the override is set to white!

This is certainly a trap for new, or old, Revit players.  If you read this blog, you are prepared.

If you don’t, you may be pranked by some White Line overrides in the very near future 🙂  Let the shenanigans begin…