Test it with:
setxkbmap -option compose:menu
Try compose, 1, 2 for ½.
Compose, ^, 3 for ³.
Compose, A, : for Ä.
It's pretty intuitive for the most useful characters, and easily the fastest way I have of typing the ö, ñ and å in various colleagues' names.
I don't yet speak the language of my adopted country, so it's better for me to keep []{} etc where I like them in the British layout, and use three keypresses for typing the ø in a (place) name like København.
If I do end up typing lots of Danish, I'll probably map AltGr+A,E,O to Å, Æ, Ø. É is rare, so I'll still use the Compose key for that and German / Swedish names.
It converts latex to unicode where possible. It's pretty impressive how much of Latex can be replaced with Unicode today.
This program translates LaTeX markup to human-readable Unicode when possible.
Here's the default text from that webapp:
Basic math notations: ∵ A͡B + B͡C ≠ A͡C ∴ ∬∜x̅ ξᶿ⁺¹ - ⅜ ≤ Σ ζᵢ ∴ ∃x∀y x ∈ Â
Easily type in hundreds of other symbols and special characters: , ℵ, Œ, ⇊, etc.
Font styles support: 𝔹𝕝𝕒𝕔𝕜 𝔹𝕠𝕒𝕣𝕕 𝔹𝕠𝕝𝕕, 𝔉𝔯𝔞𝔨𝔱𝔲𝔯, 𝐁𝐨𝐥𝐝 𝐅𝐚𝐜𝐞, 𝓒𝓪𝓵𝓵𝓲𝓰𝓻𝓪𝓹𝓱𝓲𝓬, 𝐼𝑡𝑎𝑙𝑖𝑐, 𝙼𝚘𝚗𝚘𝚜𝚙𝚊𝚌𝚎.
Now type in this box and try it yourself. ⌣̈