If you're missing games, or want them in their original packaging, or maybe just want to own the props from teh games, look no further than Ye Olde Infocom Shoppe. This guy loves his Infocom, and I don't know about you, but I'd much rather buy from someone who knows what he's dealing in than some Mom cleaning out her son's room and selling it all via eBay.
If you're looking for info on Infocom games, try Peter's Infocom Page (he even has some of the game files for download). Man, all this nostalgia reminds me of the time in the early days of the net that I tracked down a bunch of the Infocom designers and programmers, and sent them fanboy email. A couple were even polite enough to respond. Remember the good old days, when a celebrity receiving fan e-mail was something rare?
If you want to write your own text adventures ("interactive fiction" it's now called, as people are writing everything from scifi humour games to surrealist experiences) look no further than Graham Nelson's Inform. It's a language and a compiler, which spits out Infocom game files.
In case you're curious, the game files are programs for a virtual machine, the Z-machine. Infocom was inventing machines that didn't exist a decade before Java was even a twinkle in Sun's eye. There are Z-machine interpreters for almost every system (Rezrov is the Perl z-machine interpreter). I wonder how long it will take someone to make a Perl compiler backend for Z-code, so you can write adventure games in Perl?
Uhoh, I think I just lost Simon his weekend