Assuming you have OL2003, there are two separate features that nearly do
what you want:
Auto completion: Go to Tools>Options>E-mail Options>advanced e-mail options,
and check "Suggest names while completing to, cc and bcc fields". That will
suggest recently used names as you type (NOT all names from your contacts
list). To accept a suggestion, click on it, or tab to the next field to
accept the name at the top of the list. To reject the suggestions, either
keep typing, or click in another field.
Auto name checking: Go to Tools>Options>E-mail Options>advanced e-mail
options, and check "Automatic name checking". This won't give you a list,
but if for example you just type "bo", and don't accept any of the
autocomplete suggestions, it will underline the letters in red after a
while, and right clicking on them will give a list of matching addresses
from your contacts, OR you can just hit send and it will prompt you with a
list.
The two features can be used together or separately.
HTH
Mark