This is another simple and old tip with explorer. Pretty often, I come across a file with unknown extension which I want to open in Notepad. You have an Open with... option when you right click on a menu, but unfortunately it is a little slower than what I would have liked it to be and take a couple of extra clicks which I don't like.
Here is what you can do (a good old trick)... Right click a file and click on Send To -> Notepad as you can see below. In fact, if you are a keyboard lover as I am, you can use Shortcut key on your keyboard and press on N twice. The file will be opened in Notepad. I find it pretty cool since I have to deal with multiple file types while troubleshooting issues and the keyboard Shortcut key + N + N just saves me a few clicks every time.
Now, by default you won't get Notepad in the Send to menu. Here is what you can do to get Notepad (or any other Program) to show it there.
1. Navigate to C:\<WINDOWS Folder>. Right-click on Notepad.exe and click Copy.
2. Now go to C:\Documents and Settings\<Your Profile Name>\SendTo, right-click and select Paste Shortcut from the menu.
3. If you want to use the shortcut (Keyboard Shortcut Key + N + N) you should rename the shortcut you just pasted to Notepad.
You are all set. I hope you find this useful (in case you are not aware of it already) 
Until next time 
Rahul