This classic daily office humour comic recently had a website overhawl. If you work in an office, especially cubicle based - this comic is an amusing read. Dilbert comics frequently have trends which quite often map to events in the office - so true to life.
xkcd is a plain yet incredibly amusing comic. Its for the real geeks out there, with some classic amusing comic strips. They also made a lovely map of the internet, which is surprisingly accurate in terms of numbers of hits. Hover over each days comic for an extra comment to the comic, most are witty and amusing and add something to the comic.
Moving onto the more arty comics, Questionable Content has an extensive back story. It is well worth starting from the first comic, and working your way through all of the 1500 or so comics to get upto date. QC puts up comics each weekday!
A frequently dark comic with a different yet interesting style. Comics appear seemingly randomly, not always a great read in the office due its potentially offensive nature. Don't let that detract you from reading it though. This is another comic which adds an amusing tooltip to each comic, don't miss them!
An interesting line of comics with a back story, frequently comics about current events, almost always an amusing read.