What they said. Then, on top of that, –
a) you can use DataViz products to convert actual file formats for those occasions when you DON’T have a corresponding application. On the Mac, MacLink Plus lets you convert Lotus 123 or Quattro Pro spreadsheets to Excel format, WordPerfect or WordPro word processor documents to Microsoft Word, AppleWorks, Nisus Writer, MacWrite Pro, etc. formats. On the PC, Conversions Plus lets you access Macintosh-formatted floppy disks (and possibly Zip disks and hard drives, although I’m not sure about that) and also convert Mac formats like AppleWorks, NisusWriter, or MacWrite Pro to MS Word, Lotus WordPro, or WordPerfect; it lets you assign missing 3-letter extensions based on Macintosh FILE TYPE and FILE CREATOR codes, which it can read; and it lets you go the other direction and assign FILE TYPE and CREATOR to pc files you are putting on a Mac-formatted disk.
b) Most graphic formats are x-platform, but to handle the more esoteric ones you’ll probably want a program that works like a graphic Rosetta stone. I can’t recommend a PC product, but on the Mac there’s nothing like GraphicConverter (shareware). If you need to access a “windows cursor” file or a Mac PICT resource or a Windows file icon or a Macintosh image file clipping and convert it to a format supported on the opposite platform, GraphicConverter can do it.
c) Aladdin software makes nice expanders for both platforms that can unstuff, unzip, unBinHex, unTar, unUUencode, unPack, unCompactPro, unDiskDoubler, unLHA, unGZIP, and otherwise decode and decompress compressed file formats, and even mount disk images as virtual floppies. It even seems to know how to reach past “self-extracting archive” .exe (or Mac application) “wrappers” to expand those too.
d) The closest equivalent to VirtualPC to run in the opposite direction is Basilisk-II. It doesn’t emulate a modern PowerMac, it emulates a Quadra (and you have to supply the ROM file, which you extract to floppy disk by running a little utility program on the ancient Quadra that your boss still has in the office supplies closet); however, it emulates one hell of a fast Quadra (using the modern PC processor to push around the MC68040 instruction set and create a hardware abstraction layer that the MacOS sees as Mac hardware). You can run System 7.6 for sure and probably 8.1 on it, and therefore use Mac programs that will run on non-PowerMac hw such as (older versions of) ScriptEditor, GraphicConverter, or SuperCard.