My machine at home has 384MB of RAM. This is really nice when I'm running Photoshop or doing some sloppy animation stuff. However, when I'm making software that other people will use, it's a liability -- it lets me be sloppy and write code that sometimes crashes (!) in low-memory situations.
I wrote EatMem to let me simulate a smaller machine on a bigger one. It uses simple Windows APIs to lock memory (i.e. it can't be paged out) to see how apps run in low-memory situations.
Download: eatmem.zip [16kb] [source + .EXE].
New: "Phil" -- a program to help eat disk space too: phil.exe [44kb]