Unicode gives you access to some 32,000 symbols and characters, but your keyboard is still stuck in the 1950s (not that a 32,000 key keyboard would be very practical). Getting access to all those characters can be a chore, but the $25 WizKey aims to simplify it. How does it work? Short answer--pretty well, but with a few quirks.
Wizkey can appear as a long menu of characters, summoned from its resting place by the invocation of "ctrl-ctrl", and you can then mouse through the list to find the character you want. In addition, if you have memorized (or have a list of) other combinations, you can drill down much faster--ctrl+~ (tilde) brings up the tilde-covered letters, such as ã. The other mnemonics are about as easy to remember as they can be.
Drawbacks? The biggest is that there doesn't seem to be a way to "type" in Unicode -- you must reinvoke WizKey after each character. This contrasts with free/donationware Easy Unicode Paster, which overlays a virtual keyboard on your screen and lets you key in long strings. On the other hand, WizKey is virtually invisible except at the moment you're using it, and once you're memorized the key combinations--and, odds are, you're only using a small set of Unicode characters with any frequency--it can be quite speedy.
So which is better? Well, that's up to you. With Easy Unicode Paster as donationware and a 30-day trial for WizKey, it should be easy to determine which suits your needs best.