TheBrain
From the PB forum post here.

So after watching one of the Webinars a while ago, I sat down to thinking about my usage of types and tags and solidify what the heck I used them for. I had the hardest time with Types. What the heck should my types be? What do I use them for?


Types

The definition that really clarified Type usage for me is:
What does this thought represent?

This issue really doesn't occur with files. In a files case, a file is always a file. But in PersonalBrain, a thought can different things, but only one at a time.

Your types will differ, but here's a few of mine:

Achievements - Be they Workouts, eating healthy or what have you, any time I do something I want to track, I prepend the thought with the date and then what occurred. For example 2010-03-31, Running
Application - I use PersonalBrain to track software and companies associated with them, documentation, etc. So when the thought represents an application, it gets that type. I also have a Games subtype.
Category - Any thought that categorizes child thoughts gets this type. 
Company - For thoughts that represent companies.
Date - I have thoughts for each week in the year. These thoughts are typed as Dates.
E-mail - Any time I pull an e-mail in from outlook, it gets this type.
Government - Related to government forms, and specifications. It contains the subtypes Form, Mag Media and Tax.
Item - If something doesn't correspond to a pre-existing type, I call it an item. Think of it as a generic subcategory. An example is if I list a Wineries different wines as children. They all get the type Item.
Link - Any thought that represents either a hyperlink, or a link to a document. Thoughts with another type may also have links (for example, project thoughts), but this type is for thoughts whose sole purpose is to point somewhere else.
Media - I have subthoughts under here for Article, Book, Image, Music, and TV / Video / Movie. For example, The Wall would be typed as Music, as it represents an album.
Meeting - Self explanatory
Next Action - For you GTD folks, this is what I use to delineate actions.
Note - Think stickies. This is for a thought that contains text that I want to stand out from the original thought. Yes, the thought type color is Yellow. 
Person
Place
Project
Project - Inactive
Reference - This is a tricky one, and took me a while to define. It usually points to a document or link, but what defines it as a reference thought is if the thought contain documentation or referential information condensed from another source in the notes for quick access.

These types usually have colors and icons, except for the Item and Link types. This allows me to easily see what a thought represents under a parent thought, and pick them out quickly. See the attached image.

Name: Types.png, Views: 0, Size: 12.80 KB

Tags

So what do I use tags for? I use tags for action related things. Most of my tags are GTD related. For example I have a @currently and @today tag pinned to the top of my plex. I have an Inbox tag up there also, so I can easily pull stuff into PB without having to file it. If I need to copy a thought to another brain, I tag it with updated. I also have a @home and @work tag, which I use in conjunction with actions. Some people go further into the contexts, but I found I didn't use them, so I stick with what works for me. Thoughts with calendar events get a @scheduled tag. When I'm waiting for someone to get back to me, it's @waiting for.

A great example of Type / Tag interplay is my @NA Someday Maybe tag. This I mark for stuff that I want to buy, or do, or are interesting to me at a future date. I have attached an image below, but if I click the @NA Someday Maybe tag, I can see everything sorted by what the thought represents. Applications are the cog wheels, media is always pink, next actions and projects also stand out.


I hope this helps you in your own types and tags system. Much of the inspiration has come from other forum members over the years, with a recent impetus of Omar's excellent Mastering Your Tasks and To Do's webinar.
With a carefully implemented type and tag system, you'll find your brain is much easier to use.