Is there any 3rd party app to enable special character input on holding key for Snow Leopard same it is at Lion?
Thanks a lot.
Unfortunately no, I was also looking for this.
The closest I found was going to System Preferences, then International, then in the Input Menu tab, select Character Palette.
When you open the Character palette, you'll see on its left the categories of characters. I often work with mathematics so the Math and Greek categories are useful to me. If you click on the bottom Character Info, it will show all related characters to the one you selected above in the palette.
e.g. if you choose the Latin 'u', you will see ü, û, ù, µ, etc. Double-click on any of those and it will paste itself on your working document.
Hope this could (partially) resolve your question.