Recently updated, iLaugh [iTunes link] for iPhone is the go-to application for all things jokes. This simple application gets jokes from several different sources and allows you to get your laughs anywhere.
You can choose from either gcfl.net, which allows you to read some clean jokes; jokes.com, which can include some explicit jokes; there's also bash.org, which allows you to read funny IRC (Internet Relay Chat) quotes.
If you're looking for a fun application that could provide you with unlimited chortles, then this is the application for you. You can choose between a lite version and a paid version. The lite version provides you with the same functionality, except it includes some ads. The paid version is $1.99US.
Update: The developer has let us know that today there were two new sources added to both the iLaugh paid and free apps. The two sources include Chuck Norris facts in both English and French.