OpenAppMkt is a 3rd party app store like Cydia, but since it is home only to applications written in HTML, it doesn’t require you to be jailbroken. There are both free and paid apps available, and it seems very developer friendly. HTML can be used to create powerful web apps that are almost indistinguishable from their Objective-C counterparts, and has long been used as a way to circumvent Apple’s approval process.
OpenAppMkt was just opened today, but it already shows a lot of potential. To install it, just hit the source on your iDevice and tap “Add to Home”.