comic strips - such as dilbert, peanuts. not sure but i'm sure you can find some if you look around.
american comics - such as xmen, spawn. i know marvel comics offers up a subscription plan for access to digital copies.
manga (japanese) - there are free sites for downloading non-us released (fan translated) manga such as mangafox or animea. not sure about for released content though.
just google it. put what you are looking for followed by digital copy and you might find a few good results...
ie: "marvel comics digital copy"
as for "what" you should read... it depends on what type of comic you mean and what type (read: genre) you are into.
the only american comic i've heard was a few books of spawn which i found to be okay. personally i'm not a fan at all of american work (as its so bland and overdone).
as far as comic scrips...dilbert is sure to please any office worker. i've read quite a bit of it. if you're into dark humor farside can be a pleaser as well.
in terms of manga...theres too many to name. most offer much much more than american comics do. the popular three are bleach, one piece and naruto. though, there are plenty of other options as well.