I used to use PopChar as well, but these days I don't think it's necessary. I just use the character viewer (default location is top right next to the time and (on laptops) battery life indicator, same menu as for switching between keyboard settings). For most mainstream applications in most standard fonts, most of the options in the character viewer simply appear by double-clicking them, and there is a huge range of different alphabets, more or less every known accented Latin character, IPA, fractions, you name it. However, some fonts are limited in their scope and some applications don't really support the viewer at all (QuarkXPress in particular, for which you need to purchase the extended font families at inflated expense).
Regardless of how you're importing text, doing it one character at a time can get a trifle tedious if you're trying to type an extended passage in a different alphabet. To get round this in Cyrillic and related alphabets used in the former USSR, I use
translit.ru, where you can type in Latin and it appears on screen in Cyrillic. One thing to be aware of: it doesn't accept keyboard shortcuts for things like cut and paste. If you hit ctrl+C or apple+C you will wipe what you just wrote. Use the mouse and right-click to copy and paste. It also gets a trifle confused if you use it with a keyboard setting other than English (it doesn't really know what to do with the letters Å, Ä and Ö, for example).
I would imagine similar tools exist for Greek and other alphabets.
