Today, I've played a bit with printing right from iOS devices (iDevices for short).
As you may know, Apple's own solution, AirPrint, is compatible with only few printers, all from HP. (Fortunately, no longer only the [in my humble opinion] lowly and unreliable inkjets are supported, but also LaserJets, but still: there's still no support for other manufacturers' gear and not even HP's own, older models.)
Needless to say, the two printers I tried to print to, a Samsung MI-2525w and an HP LaserJet 1320, aren't supported either.
As far as the HP printer is concerned (there's no such app for Samsung printers), there's an HP app supporting it in the AppStore, “HP iPrint Photo”. It allows for printing photos and documents (like PDF files). It's a sandboxed app (as with all the other AppStore-based printing apps), which means it doesn't add support to stock apps otherwise capable of (AirPrint) printing. Printing images via HP iPrint has always resulted in a cropping of some degree – about 10-15 pixels (on a hi-res iPhone 4) of each side was cropped. This was sufficient to completely hide the leftmost part of the letter “L” in “Locked SIM” (on an iPhone 4) or the “i” from “iPod” (on an iPod touch 2G).) In addition, it's REALLY slow and unreliable – in many cases, the task (both simple screenshot images and a two-page PDF file first transferred from Mail) I've sent the printer just didn't get printed. (Actually, I was never able to get my test PDF file printed. It got, generally, printed just fine from TruPrint but not from HP iPrint.)
A real upside of the app is the ability to print on hardware / firmware versions not otherwise supporting AirPrint. I've tested with the 4.2.1-based iPod touch 2G. (4.2.1 only allows for printing on at least (true) third-generation devices like the iPhone 3G S but not the second-generation iPT or iPhone 3G – unless you allow multitasking on these models as is explained HERE)
As of version 3.0, it completely lacks in-app “New document” capabilities, where one could just paste the contents of a mail or webpage, even with losing formatting, for instant printing. (Pasting text to local, in-app notes is supported in several other, commercial AppStore apps, for example, ePrint by Microtech Corp. [more info HERE]; that is, there's no such problems with those apps. Of course, only having access to sandboxed documents will always mean losing formatting when you copy e.g. mail or Web page bodies.) It doesn't have iTunes File Sharing capabilities either, but that isn't that big a problem (after all, if you have access to a desktop, why not printing from there instead of transferring the files to be printed?)
Speaking of ePrint by Microtech Corp., it heavily lacks file exchange support. That is, you just can't pass it a PDF file from mail – unlike HP iPrint. Nevertheless, it prints (textual) notes (non-formatted content passed via the clipboard) just fine (and much-much faster than HP iPrint.)
The best solution I found was Cydia-based (that is, jailbreaking required!) TruPrint (more info HERE; developer page HERE), which, unlike the previous two applications, is not a sandboxed one. Upon selecting the AirPrint menu item anywhere (Safari / Mail etc.), it will be invoked, listing all the available (and before encountered) printers and letting to print to them. It worked wonderfully with both the Samsung and the HP model (tested with both formatted text and the two-page PDF test file) without cropping or refusing to work at all. At least initially; after some 1-2 hours of continuous switching between the three apps on my 4.2.1 iPhone 3G S and 4.3.3 iPhone 4, it no longer would print the test PDF file. Before that, however, it was the fastest, most compatible (with both printers and existing apps / formats) and, software-wise, best.
It also has a trial version; as the full title costs $9.99, you will want to give it a thorough test ride to see whether it works reliably in your particular configuration.
An example screenshot of all this: