File management and printing
There are lots of Wi-Fi file-sharing apps for the iPhone, and many productivity apps include such "air-sharing" capabilities. Avatron Software's $3 Air Sharing is a good option if the built-in file-sharing in your other apps isn't enough. So is the $5 ReaddleDocs, which lets you collect Office, iWork, and PDF files from the Web, email attachments, and other sources for viewing, with extra capabilities such as extracting text from PDF files.
DigiDNA's $5 FileApp Pro is a file organizer for your iPhone. With FileApp Pro, you can copy files to and from your iPhone, then organize, rename, move, and multiple-select files, as well as open them in compatible iPhone applications.
Chances are you don't yet have an AirPrint-compatible printer, so the new iOS 4.2's print feature isn't usable. But you can print to many Wi-Fi-connected printers using an app such as EuroSmartz's $8 PrintCentral; it has you copy what you want to print into the app, which then prints that material. It also allows you to print to a network printer across your Wi-Fi network. (To print via your PC or Mac, such as to access non-Wi-Fi printers or to print via your computer over 3G, you need to install EuroSmartz's free WePrint printer server on your computer.) Other iPhone apps that can print directly to Wi-Fi-connected printers include EuroSmartz's $9 Print n Share and Wellala's $3 Print Magic.
Cloud storage and FTP
Box.net and Dropbox are cloud storage services that offer free access to limited file storage and charge for additional storage. Their iPhone apps let several applications get files from and put files on its online storage server for access by your other devices and computers.
Apple has its own MobileMe iDisk cloud storage service as part of its $99-per-year-per-user MobileMe service, which includes an email account and online calendar. The MobileMe iDisk app lets you exchange files directly with this service.
The Google Docs online software toolkit lets you work on text and spreadsheet documents on the Internet from your browser. Unfortunately, it is very difficult to use on an iPhone. Although several apps let you open documents stored in Google Docs and send documents to it, the $4 GoDocs app from LightRoom takes a step further and lets you actually edit those documents directly from your iPhone. (You can also edit them in Google Docs-compatible apps such as Quickoffice and Documents to Go.)
If you work with your own or clients' websites and need to send files to and from, Headlight Software's $10 FTP On the Go Pro will do the trick. It also lets you edit Web pages and related files, so you can update your site when on the go from your iPhone.
The free Citrix Receiver lets you run Office, SharePoint, and other data center apps on your iPhone, through a virtual window to your Citrix-enabled servers. You, of course, need Citrix set up on the back end.
If you want to run applications on your Mac or Windows PC, or on remote servers, from your iPhone, consider Wyse's $15 PocketCloud. It allows remote control of a PC from your iPhone.
iCacti's $4 Server Monitor lets you monitor usage stats graphically for your Windows, Linux, and Mac servers, as well as network routers and printers. With Harlekins' $12 Server Admin Remote, IT staff can monitor Mac OS X Servers from their iPads.
The $30 LogMeIn Ignition lets you remote-control your Windows PC or Mac over the Internet. Another way to remote-control a PC or Mac to run its applications from an iPhone is with the $6 Mocha VNC app. Hana Mobile's $5 Connect to PC is aimed at people who need to run just Windows desktops remotely from their iPad. Hana also offers the free Connect My Mac app. In both cases, multiple connections are allowed.
If you need to remote into SSH- and Telnet-based computers, Zinger-Soft's $10 iSSH can do the trick.