VIDEO PLAYERS
 
RealPlayer (.rm, .ra, .ram)
Windows Mac Linux
Real Video is the most common compressed format for anime on the net. The player, Real Player 8 Basic, can be downloaded free from the Real Networks homepage. although a more feature rich version can be purchased for $30 US but this doesn't offer a great deal more than the free version, particularly if you're only going to use it to watch fansubs.
 
Microsoft ASF (.asf)
Windows Mac Linux
Microsoft ASF (Advanced Streaming Format) is a new video format that has gained widespread use only in the last year or so. ASF files are arguably about on par with Real Video quality for equivalent bit-rates. You can view ASF files using Windows Media Player which can be downloaded for Windows and Macintosh (bad luck linux users).
 
DivX (.avi)
Windows Mac Linux
DivX is an MPEG-4 codec which provides incredible quality video, often better than MPEG-1, for around the same size as an ASF file, typically 500kbps. Unfortunately this comes at a cost, with the minimum hardware requirements being greater than that of most of the other video formats described here (although not that much greater than ASF). I would say you probably need a 300Mhz+ processor to get decent performance. Here's a tip for those using Windows Media Player if your suffering from poor performance. When playing a DivX file goto File menu, Properties. Select the Advanced tab. Then select "DivX ;-) Hi-Res Video Decompressor" and click Properties. Here you can adjust the quality of the playback. Set it all the way down to zero for smoother playback.

Happily DivX codecs can be downloaded for Windows, Macintosh, Linux, and beOS.

 
MPEG-1 (.mpg, .mpeg)
Windows Mac Linux
The MPEG-1 format has been around for a while and is the format that is used for Video CDs. Despite it's age, the use of MPEG-1 has actually become more prevalent in recent times as the average bandwidth of end-users has increased. MPEG-1 is the nicest looking of all the video formats described here, but you pay for it in terms of file size (MPEG-1 files are typically encoded at around 1,500 kbps). Windows Media Player works fine for playing MPEG-1s and it should be fairly easy to track down players for other non-Microsoft platforms.
 
VivTV (.viv, .vivo)
Windows Mac Linux
Vivo is another format used although it's not so commonly used now days. There are a number of Vivo players around, but the best by far, in my opinion, is VivTV by a group called MediaTV. Not only is VivTV freeware but it also supports a large number of different display modes.