As an engineer, I like Ubuntu's strong repository (has Scilab and software for programming microcontrollers) but would prefer something more lightweight, especially since Ubuntu ends support of older versions while making newer versions more power-hungry. With each revision, some users of older computers get cut off and forced to another distro.

I tried Debian, but it requires MUCH more tweaking to get it working, and it takes a LONG time to install it. I've been spoiled by Puppy Linux (so user-friendly and so lightweight), but it's software repository is weak, and I had difficulty with the Woof system (perhaps it will be better by 4.3.2 or 4.3.3).

So what other Linux distros have a good repository like Ubuntu and are at least as user-friendly but are lighter-weight?

Linux Mint and Crunchbang are both based on the Ubuntu Kernel and use it's repositories. Crunchbang uses the Openbox Desktop environment and is much more lightweight.

Well, I am in a similar position to you, but have come to a position of some equanimity about it:

• If you didn't install it, you wouldn't have it, and so your installation would be lighter weight. Knowing this, doesn't stop me installing things on my laptop "just to try it out" or "just in case". But I do that, knowing that it is in opposition to my general goals.
• If you want lighter weight, you want a lighter GUI. Trouble is, when I try it, I miss the labour saving gee-gaws of KDE, etc. Enlightenment is very nearly wonderful, though.

Ubuntu minimalistic install (server) and fluxbox as wm.

Spending a little extra time to get Debian configured is well worth it. I bet you can find a detailed installation guide (or maybe even a youtube video!) to walk you through the install process. It should not take more than an hour to install a working Debian desktop from scratch. I would recommend Xfce or LXDE desktops if you want something a little lighter than the default Gnome. Good luck!