iOS: Prompt is an excellent SSH client for iOS, providing you with an elegant interface and several great features (like SSH-specific autocomplete) so using the command line on your iDevice is actually a pleasant experience.
The are a large number of SSH apps for iOS, most of which will cost you at least $1.19 and as much as $25. While Prompt isn't the cheapest of the bunch (it's $US5), it is a step up from many of the others we tried. One of the main things that sets Prompt apart is its interface. While you probably wouldn't think much about interface when it comes to a command line, it matters on a touchscreen device because you're not dealing with menus, keyboard shortcuts, or an external keyboard (unless you're using one via Bluetooth). On a computer, the command line can simply be a command line. On an iDevice, it requires an interface to function. If that user interface is distracting and difficult, you lose the simplicity of the command line.
Prompt's interface is minimal and gets out of your way, where virtually every SSH app either doesn't or hides everything so much that it's difficult to access. While there's nothing wrong with the iPhone/iPod touch version of Prompt, the iPad version is ideal because of the larger screen size. Fortunately Prompt is a universal app, so you get all versions when you buy it.
Interface aside, Prompt has some really awesome features. It is capable of autocomplete, supports keyfiles, lets you customise your special keys (you just tap and hold down to switch them), and supports special keys on Bluetooth keyboards.
You can get cheaper SSH apps for iOS, but if you use SSH frequently you'll likely find that Prompt is your best bet.