Windows only: Despite probably being a copy of Sogou’s similar product, Google’s IME is quite powerful and a welcome change to the antiquated built-in IME system of Windows.
Direct link to installer here.
See below for newbie-friendly instructions
Before you install, make sure to have have support for Chinese installed by going to Control Panel–>Regional and Language Options.
Under the “Languages” tab, make sure to check the box entitled “Install Files for East Asian Languages”
It might ask for your Windows disc to install the necessary files. After it does, install “GooglePinyinInstaller.exe.”
The IME will now reside in the Language Bar, located just to the left of Taskbar in the lower-right corner of Windows.
To type in Chinese, click on the “EN” button 
to bring up the language selection menu:
Choose “Chinese (PRC)”
Now you can type pinyin. The IME is smart enough to recognize words and phrases, so there is usually no need to choose individual characters. For example, if you wish to type 你好吗, you can simply type “nihaoma” and the IME will recognize it as a phrase.
Simply hit the space bar to choose the first from the list. You can also choose one of the other choices by typing the appropriate number.
If you are looking for a specific character that might not show up as one of the first few options, you can scroll through the results with the + and – buttons. Take for example the character yǐ 矣 (a Classical Chinese particle). Type “yi” and see the first results, which represent the most common characters pronounced as yi
.
To get to our yi 矣, simply scroll through the choices until you find it:
There it is, #2.
The IME will now learn from your usage and move this yi 矣 to the first page of results, so the next time you type yi, it will guess that you might be searching for this yi and not another, say 一 or 以.
To switch to traditional characters 繁體字, click on the blue gears 
on the IME bar. From here choose 属性设置 (properties setup) and then the 词典 (dictionary) tab. Finally, check the third box down, entitled 繁体模式 (traditional method).
Finally, you can toggle the IME on and off with “ALT-Shift.”
Want to learn more?
Features list (Chinese)
Enable Chinese input in Ubuntu Linux
Enable Chinese input in Mac OS X
Still hungry for more? Have a sandwich some dumplings.
[...] are the days when one had to use NJstar to type in Chinese, what with modern IMEs and such. Now we can use the word processor of choice to handle our various languages. Here at [...]
By: Poll: Which word processor for Chinese? « Electric Sinophile ⓔ華狂 on March 12, 2008
at 8:04 pm
Some of the screenshots have gone haywire in this post. I will fix them.
By: manchucka on March 30, 2008
at 10:40 pm
FIXED
By: manchucka on March 31, 2008
at 2:21 pm
[...] we’ve talked about the Google IME here (or was that the SoGou IME?), but as the resident reactionary (read: I still refuse to bow to the [...]
By: So You Want to Type in Chinese, eh? « Electric Sinophile ⓔ華狂 on May 10, 2008
at 2:27 pm