More great logos would mean more great t-shirt art and more exposure for web comics.
I have a gallery of web comic logos at my site Psychedelic Tree House. Have a look. (If you already have a logo, please send it to me for possible inclusion.)
I probably wouldn't include a few of them in retrospect, because they are page title headers disguised as logos. The fault is mine.
A great logo automatically looks great on a t-shirt. A rectangular page header, even with nice lettering and art -- not so much.
Some of the t-shirt transfers you stick in your printer and then iron on to a t-shirt are excellent. Avery transfers and Epson printers perform best. I buy my transfers in the neighborhood drug store. I have had no luck with the transfers for dark shirts, so I stick with tan and white. I like the shirts made by American Apparel.
The main trick is to get the iron nice and hot and make sure the ink is eager to peel away from the backing before pulling it away. I now have a commercial duty press and no longer use an iron, but for short runs an iron is fine. Just think, you could be wearing your comics logo around work or campus tomorrow. In full color.
A small detail is to remember to tell the printer whether you are printing a mirror image or not. If you have text, you need to print mirror image for it to be readable on the shirt.
Now that I've inspired you to make a logo, keep in mind it is useful for all sorts of communication regarding your comic. It can be part of an email signature in reduced size, your letterhead, your business cards, your portfolio, your splash page, your advertising and much more.
Read my short piece on the Psychedelic Tree House site about what makes a logo and what doesn't before you plunge in. It'll help you make something great.