Because of my job, some weeks I spend a decent amount of time on the web wasting time in very short bursts. Working with video, I do some work on my editing program and then hit "render," giving me about two or three minutes as the program converts the video into a watchable format. There's not a lot you can do in that amount of time, so webcomics are a good fit. Here's a top-five list.
Honourable Mention: Questionable Content, which works much better when you read it in huge chunks than it does on a daily basis. I remember I always felt the same about Fox Trot growing up - it wasn't worth following in the paper or online, but it was worth reading the collections when they came out. To that end, I have linked to QC's first comic, and you are welcome to start from there. I'd recommend it as an activity for a rainy day (unless it's thundering, at which point you would be wise to shut off your computer to avoid damage from surges).
Mostly I just made QC an honourable mention so I wouldn't have to hunt through its archives for a good example to put here. Also because I wanted to type "honourable" with the British spelling.
Second Runner-Up: Penny Arcade. It's all a little too much inside baseball to rank in the top five for me, but I subscribe to the RSS feed and always keep up. Gabe and Tycho have been keeping the world informed about the idiocies involved in the world of gaming for so long that their popularity has grown to the point that they are now actually making Penny Arcade videogames on the side. There's a lesson there somewhere.
Honorable Mention: Married To The Sea. A husband and wife duo who take old Victorian drawings and add new captions to the photos. A fun concept that works even better in the reality than it does in theory, MTTS is loaded with fun comics to post around. like this one:
5. Piled Higher and Deeper. This guy started a webcomic about being a grad student in 1997. Seriously, 1997. I don't know if I even really knew what the internet was in 1997*. I was still in middle school, playing NBA Live and listening to Jars of Clay. But there Jorge Cham was, working on PhD comics instead of doing his grad work (he was - surprise! - getting his doctorate at the time). It simultaneously makes being a grad student look both fun and brain-meltingly boring, as his characters seem to spend most of their time putting off work in order to go find free food. Which sounds about right to me.
*The first webcomic was Doctor Fun, started in 1993, by the way.
4. A Softer World. A surrealist view of life done in the most emo way possible, the writers will take a picture, spread it across three frames, and add mental dialogue. You've got to see one to understand:
3. Ctrl+Alt+Delete. More of a traditional comic form, it also works best as a long read on a snowy evening. It follows a completely hairbrained main character as he enacts crazy plans that ruin the lives of his more levelheaded roommate and girlfriend. Naturally, being a webcomic, it's also mostly about video games. One of the characters is actually a combination XBox/robot, which I suppose is a less traditional comic form:
2. Real Life. Loosely based around the life of creator Greg Dean, it stays true to its name as we follow Greg from 1999 - when he mostly sat around playing video games with his friends - to the present, as a full time cartoonist. In the meantime, you could follow him as he went to culinary school, moved to California and then to Texas, and fell in love and got married. Of course, Real Life only extends so far, as the character also seems to find time to destroy buildings from Star Destroyers from time to time:
1. XKCD. The clear winner. The title doesn't mean a thing, the drawing is just stick figures, and it's generally just comics about math, but for some reason it's my favorite thing ever. A former NASA scientist decided to reinvent himself and chose this as his new path. Here's a sample: