Out of the box, many things can be readily handled by most cell phones today - simple text messaging actually accommodating not only 'secret' messaging, but tracking and emailing more detailed adventure info. Higher end phones - becoming quite common now, and lower end ones with limited screen/internet access can display maps and other info. Character sheets and such are easily handled even if only as text messages, notes or on a forum post.
Content creation and rule mechanic automating is where custom software is needed and device dependency/ease of use come in to play. The former can be done on less portable/more powerful devices and hosted for free on blog sites and free hosting sites (wordpress and webbly for example).
The later, for those for whom native programming is not feasible, can rely on others or make use of script/spreadsheet-style applications.