In TWClassic (aka TW2), the was a default (shadow) tiddler called "MainMenu" that appeared on the left.
In TW5, the default layout doesn't have a MainMenu, but you can create one relatively easily, like this:
1) Create a tiddler named "MainMenu"
2) Tag it with "$:/tags/PageTemplate"
3) Create a field named "list-before" (leave the value blank)
4) Enter the following content:
<div style="position:absolute;margin-top:1em;">
Stuff Here
More stuff here
</div>
5) In the $:/ControlPanel, Appearance, ThemeTweaks, set the "Story left position" to leave room for the desired width of the MainMenu (e.g., "20em")
(note: you can also directly set this value in $:/themes/tiddlywiki/vanilla/metrics/storyleft)
That's it. You can now edit the MainMenu tiddler to show whatever content you like.
Notes:
* If you want the MainMenu to NOT scroll with the page, use "position:fixed" instead of "position:absolute"
* If you want the MainMenu to wrap its contents to fit the width, add "width:20em;"
* Adjust the "margin-top:1em" to suit your taste
enjoy,
-e