One of the under-the-radar killer apps of Apple's iPad is the collection of comic book storefronts (which also double as comic readers). They're all free, and connect you to virtual comic shops that sell digital issues of comics for about $2 each--with transactions handily conducted through Apple's app store back end.
There are official Marvel and DC Comics apps, along with indie apps such as Comics + app from iVerse, IDW Comics, and the Comics app from ComiXology, which sell titles from a variety of large and small publishers. All of these apps have a very similar layout, and many sell overlapping products. Even better, they all have a frequently changing collection of free comics available to download, making it easy to build a small virtual comic book library for the low, low, price of free.
As a former comic reader in my high school days (I know, hard to believe, right?), who has done little-to-no comic book reading in the past 20 years, I was surprisingly sucked in by these apps, but also found the free offerings available varied wildly in quality, from full issues of classic comics, to skimpy "preview" issues only a few pages in length, to low-grade indie comics that look like they were drawn in the back of a grade school textbook.
To the end, I took it upon myself to sample the free wares available in the biggest iPad comic book apps and pick the best examples for casual readers. If you've downloaded some free winners not on my list, let everyone know about them in the comments section below.
Note: Some of these free comics come and go at a pretty quick clip, so they may not still be available (or may no longer be free) when you look for them -- the Marvel Comics app seems to have especially high turnover.