As twerppoet suggests, PrintCentral is the best third party app for printing. It can print directly to most WiFi printers. Best of all, it cleverly 'inserts' itself as a valid AirPrint printer into the Print menu of those apps that support in-app printing under iOS 4.2. As you probably know, Apple introduced a semi-proprietary print protocol called AirPrint recently and, at the moment, there are relatively few printers that support it. The Epson, for example, is not supported.
But PrintCentral fixes this problem by (sort of) translating the AirPrint protocol into a format that non-AirPrint printers understand. Best of all (and unlike many other third party printing apps) it does this from within the app that you want to print from. Many (all?) of the other printing apps require you to leave the original app to print. In PrintCentral, as I mentioned, you don't need to do this and can print from within the app.
So, for example, in Apple's 'Pages' word processing app, if you select 'Print' you'll see a list of any AirPrint printers that the iPad can 'see' (probably none!) and, if you've installed PrintCentral, an option to 'Print using PrintCentral'. Select that and you're done!
A couple of points
(a) you need iOS 4.2 to use in-app printing with PrintCentral
(b) PrintCentral needs to be running in the background (iOS 4.2 is the iPad's multi-tasking OS, so PrintCentral can be running at the same time as, say, 'Pages')
(c) you need to initialise PrintCentral first of all (it's a once only activity, where you let PrintCentral 'find' your printer). PrintCentral supports most (but not absolutely all) wireless printers. But even if your printer isn't supported *directly* they have a free-to-download app for your PC or Mac called WePrint that allows you to print to *any* printer via your Mac or PC (assuming you have a wireless network connection on your PC or Mac). This also means that you can print via wired printers that are connected to your Mac or PC.
PrintCentral has a number of other features too. The one I like the best is the ability to wirelessly transfer files from your PC to your iPad (or the other way around) without using a cloud server or iTunes.
Tim
Scotland