I had Windows 3.11 for Workgroups tweaked using the following (still miss that setup, though it's probably like missing high school - if I went back I'd probably hate it!):
FMStepUp
FMToolbar
ToggleMouse
PLUG-IN for Windows
Instant File Access
The only two that still run udner Win2k/XP are FMStepUp/FMTToolbar and ToggleMouse (development only stopped a year or two ago and the one-time payment that I made for a license was good right up to the end), though I no longer use either. I never made the move to Win95 because I had 3.11 setup so nicely. The little notebook saw me through a Master's degree in England, an idle nine month stretch back home in Canada (95/96 - did all my web surfing and e-mailing with it) and only gave up the ghost in South Korea in 1998 when the basement flat I lived in got flooded in the August rains and the powerconverter was immersed. I could have had it repaired (and really wish that I had) for about $45, but I gave it to a school teacher to use in class - complete with five years of work and e-mail still on the harddrive! I'd never do that now and can't think why I did it then.
Oldest software that I use now...Windows XP Pro

PS Forgot about an app that used to give me long file name support.
It *might* have been built into Plug-in or FMStepUp. No longer remember but it was indispensible. Just went digging and it was actually Instant File Access... Either PlugIn or FMStepUp was reengineered for Win95 (and 98?) but by the time I got to Win98 it had disappeared. I do rembember running a demo of it on a work machine that had Win95 on it...