I'll have to say... the turbo switch and the hardware keyboard lock. (see attachment in previous post)
Yes! I used to have a keyboard with a turbo key, and turbo+F11 was awesome! Locked the keyboard so you could do a good job cleaning it without shutting the computer down.
I also miss McAfee Office 2000, but only for 2 things: Crash Protector and Nuts & Bolts.
Crash Protector added an extra button to the poor excuse of a "task manager" in Win98: a "reactivate" button to kick a not responding program back from the dead so you can save your work and exit nicely.
Nuts & Bolts had the best defragger I have ever used. It was lightning fast and quite thorough, unless you chose the option to 0 the free space, which then it was just thorough. I really wish they would have continued the development of that. I really miss it.
A couple more things I miss:
Crystalport browser: that was the power users browser, with tabs and popup blocker, long before other browsers had them. And it had a few unique features I haven't seen in any other browser yet, like channels, which created subtabs to a main tab, that could load all the bookmarks of a particular category under a single tab. And AppCapture, which allowed you to run any Windows application in a tab.
Big Fix, back in the days when Tucows was THE place to get software. It was as close as you would ever get to a package manager for Windows, watching your OS and installed software, comparing your installed patches to what Microsoft had available and letting you know immediately when anything new was released, with a much better explanation of what the patch was for than Microsoft supplied to end users. It also made sure your antivirus was always up to date, back in the days when you had to manually download and update the dat files, yourself. And if a newer version of any of your installed software popped up on the Tucows site, you were immediately informed about it and could download & install it through the Big Fix application. You could even browse the Tucows site through it and select more software to install, while you were waiting for the huge update files from Microsoft to download on your dialup connection. If your PC manufacturer had a contract with the Big Fix company, you even got driver updates. It also reminded you to do things like defrag, clean out your temp folder, scan for viruses, and even made some suggestions for settings changes to make your pc a little more secure, like not hiding files and folders, and known file extensions, and opening certain file types in notepad by default (.js and .vbs). And all of this in a freeware app!