What is manga
Manga is the Japanese word for comics and print cartoons. Outside of Japan, it usually refers specifically to Japanese comics.
There are many active fansubbing groups out there translating the various mangas (and not just from japanese, but also from korean and chinese) and the translated scans of these mangas are called scanlations.
Manga & Anime
Chances are that if you like anime, and don't have any strong feelings against reading, then you'll like manga.
Many popular anime series are actually adaptations from manga series though the faithfulness of the transposition from manga -> anime varies widely. In some cases they are very similar (eg hunter x hunter), in others most episodes are similar but there are occasionally original storylines (e.g. detective conan & naruto) and in some cases it seems like all they've kept is the name of the story and maybe the characters.
Why bother with Manga
There are various advantages associated with manga over anime, like smaller file size, number of series available and the speed you can go through storylines. Manga is also good if you can't wait to see whats going to happen next in your favourite anime as the manga is usually quite far ahead. Manga also often goes on much longer than anime does, e.g. Berserk is still going, so you get to find out what happens next if the anime leaves the ending kind of open.
Bottom line is you should give it a go because you might like it.
Where to get Manga
By far and away the best source i've found is lurk which uses IRC to distribute files from various bots and they have about 10000 files available made up of a mixture of chapters and volumes (groups of chapters).
#lurk (the lurk IRC channel) can be found on IRChighway and the main 2 bots are mirrors (storing manga #-K) and neverwhere (manga L-Z). Once you start following a series new releases can be found on the wandypot bot. MIRC is the standard program for getting things from IRC and and if you have any problems either post below or read a guide on MIRC. As usual read the guidelines on the website/channel to see what you can and can't do on the channel (don't use @find !!)
Other sources include all the usual filesharing networks, or a wide variety of websites which detail the releases of various fansubbing groups (many fansubbing groups post info on the web and release the stuff through MIRC).
Bakaupdates lists many new releases and for reviews/ratings of mangas check out Anime news network.
Other links can be found through wikipedia and by searching google for scanlations.
Manga normally comes in a zip or rar file, so personally I'd strongly recommend you get CDisplay, as it can read direct from zip and rar files without any need for extracting them.
If you want to use your own normal picture viewer then you'll need WinRAR or some other unzipping programme (7-zip is free).
Overall my top picks are Berserk, One Piece, Hellsing and Ichigo 100%
If you like fighting stuff (referred to as 'shounen') check out 'Berserk', 'Ares' and 'Vanguard'. Another light hearted one is 'One Piece'.
If you like comedy check out 'GTO' (and its prequel 'shonen junai gumi'), 'yakitate japan' and 'midori no hibi'
If you like romancey stuff (referred to as 'shoujo') check out kare kano, hana yori dango and love hina.
If you like ecchi (don't know best way to describe this in english) check out 'Ichigo 100%', 'Gunsmith cats' and 'Love Hina'.
If you like more adult stuff (as in psychological or f**ked up, rather than porn) this is referred to as 'seinan' check out 20th century boys, blame, homunculus and koroshiya. NB the last two can be pretty weird at times!
Why write all this
It seemed to me that Manga had been ignored on this board and I thought it was a shame as its really very good. (also i have exams soon and this is a displacement activity to avoid revising T_T )