Use the box to search in the unicode table for characters.
Advanced search options:
To achieve this, this website provides a large text box where you can type text in. To type characters which are not on your keyboard, there are several ways:
The only platform independent program I know that can do the same is GNU Emacs' input method support - but downloading a full GNU Emacs for just typing a German or French or Esperanto e-mail is usually not an option (especially in internet cafés).
This quickstart manual tells you how you can use IntlTyper to type "Österreichisch, Français au Español". (Österreichisch is German and means "Austrian", Français is French and means "French", au is Esperanto and means "or", Español is Spanish and means "Spanish").
You can use the large box like a regular editor. When you press the Tab key, you will jump into the "Code" box, where you can enter mnemonics. By default, mnemonics are looked up after two characters automatically ("Short Mnemonics"). Press space to toggle the lookup mode to "All Mnemonics", then to "Unicode codepoints" and back again. "Unicode codepoints" are looked up after four characters and treated as hexadecimal unicode numbers. For example 20ac is an € sign.
Another feature of the editor are AutoType definitions. They are activated by typing certain text into the editor box (due to technical reasons, copy&paste of single characters cannot be distinguished from typing; copy&paste of longer text will not trigger AutoType definitions). They will only work if you type the last character last; if you insert a character in the middle or move the cursor in between, it will not work (this can be used to type things that usually get replaced).
There are two new AutoType definitions included: German and Esperanto. All the AutoType definitions from the MultiKey program are included as well. German will replace "Ae, Oe, Ue, ae, oe, ue, sz" by "Ä, Ö. Ü. ä. ö. ü. ß"; Esperanto will replace "Cx, Gx, Hx, Jx, Sx, Ux, cx, gx, hx, jx, sx, ux, C^, G^, H^, J^, S^, U^, c^, g^, h^, j^, s^, u^" by the Esperanto equivalents. However, since those combinations are quite common, there are "bailout" definitions available that allow you to type these keys easily as well: Either put two dashes in between (e. g. "a--e" will become "ae") or double the second key ("aee" will become "ae", "ae--e" will become "äe"). If you need two or more dashes, just use one dash more ("a---e" will become "a--e", "a-----e" will become "a----e"). Or use Copy&paste, or disable AutoType, or type it "backwards" - first type the last letter and then enter the rest before it... The MultiKey ones do not have bailout definitions; just move the cursor to avoid them.
This tab can be used to search the unicode character list. Just enter words that should appear in the unicode title (e. g. search "capital cedilla" to find capital letters with a cedilla, or "capital h with" to find capital letters h with some ornaments). You can negate words as well: Search "phi !greek" to find phi symbols, but no greek phi letters. You can add a "*" at the beginning or end of a word to search for incomplete words, like "note*" to find both note and notes or "*space*" to find all kinds of spaces.
You can also search for unicode codepoints (like "#20ac" for the Euro sign) or ranges ("#0020-007f" is all ASCII). Ranges can be typed using wildcards as well: "#01xx" is "#0100-01FF". If you want to search for a single codepoint that does not start with a letter (#A000 to #FFFF start with a letter) you can omit the # sign.
If you have a character (for example by copy&paste) and want to know its Unicode codepoint or its description, enter it alone or include it in quotes: "ä". You can include more than one character as well, if you use quotes.
The results view will show you all matching characters, their description and the mnemonics they are assigned to.