Every once in a while you need to use an old version of some software. Maybe to load an old file or in a testing environment. There is a great source for these old versions called OldApps.com.
From their website:
The Idea behind OldApps.com is ingeniously simple. Our mission is to provide our users with a wide assortment of current versions of familiar software, and their predecessors, for free. Our database is maintained up to date with older versions of software as they become readily available. Our users can be assured that this process involves our careful and meticulous screening of each individual program and version listed on OldApps.com. Our pledge is that our users will never end up with any malware, trojans, and/or any other variation of viruses.
While most web-sites provide downloads of current versions, OldApps.com caters to a different market of interest by providing older versions of the same useful programs. Often newer versions are more complicated to use and we understand that it is hard to find older, more user-friendly versions of popular software. Many software providers do not include older versions of their software on their sites, therefore, OldApps.com has found its market niche and provides a vital intermediary function for our users' software needs. We are more than happy to accommodate you with "freedom of choice" by providing multiple versions for you to choose the optimal software version that is right for you.
As software updates are being released more frequently, not every computer is able to keep up with the minimum requirements of new software; OldApps.com hopes that we can provide a means to circumvent this problem. Many users with slow Internet connections may want to attain a leaner program to avoid the common frustrations associated with the increasingly larger sizes of modern programs. OldApps.com lists file sizes next to all versions of software to ensure maximum customization. As we are expanding our library, we will diligently continue to add features to the site, so be sure to keep in touch with us.
They have Windows, Mac and Linux sections.
So if you need an old version of SnagIt for Windows NT 3.5 you can find it there. I haven't found a similar source for old DOS programs but often searching for the particular name - Lotus 123 or VisiCalc or…) will yield results.