Some of my most used apps not discussed so far would have to be
Shazaam(uses phone to identify songs, great for songs on the radio that you would like to know)
IMusic(download mp3s directly to your phone)
K9 mail - much better than stock email on moto droid
Beautiful widgets- gives your phone a great looking clock and weather widget, you can even skin it to your preferance
SetCPU - if you are rooted it let's you overclock your phone processor. With moto droid stock is 550mhz and I'm running 1ghz with setcpu it really makes a difference.
And the best for last.....
Swype - it is a beta release of the swype keyboard soon coming to android. Even in beta this soft keyboard is invaluable, it makes texting and overall typing on the phone effortless. Check it out on youtube then google it to download since it is not in the market. I am typing all of this on my droid or I would post links.