Why these codes don't need a leading zero. Windows has two parallel ALT code systems. Codes starting with 0 (like Alt+0225) use the Windows-1252 character set — what you want for accented letters. Codes without the leading zero (like Alt+3) use the older IBM Code Page 437 from MS-DOS. Most modern characters you want are in Windows-1252. But for the old-school graphical characters (smileys, hearts, card suits), you need the no-leading-zero codes.
The most famous: ALT+3 for ♥. This is probably the most-searched ALT code in the world. Ancient IRC users, early AIM chatters, anyone who wanted to add a heart to a chat message, all learned Alt+3. It still works today — in Word, browsers, and most messaging apps.
The smileys: ALT+1 and ALT+2. Alt+1 gives you ☺ (white smiley). Alt+2 gives you ☻ (black smiley, outlined). Predating emojis by decades, these were the original digital smiley faces.
Card suits: ALT+3, 4, 5, 6. The four playing card suits in order: ♥ heart (3), ♦ diamond (4), ♣ club (5), ♠ spade (6). Note that in the original IBM order, hearts come first — not spades as many modern card programs order them.
Gender symbols: ALT+11 and ALT+12. ♂ (male, Alt+11) and ♀ (female, Alt+12) were included in CP437. The same symbols are also Mars and Venus in astronomy.
Music notes: ALT+13 and ALT+14. ♪ (single eighth note, Alt+13) and ♫ (beamed eighth notes, Alt+14). Common for music-related chat usernames and decorative text.
Sun/star: ALT+15. ☼ (Alt+15) is a sun/compass-rose character, sometimes used as a bullet or decorative glyph.
These work everywhere classic. Word, Notepad, file names, email, old chat clients. Some modern web apps may strip or substitute them with emoji — but the Unicode glyph is still copy-pasteable from this page.