The Estonian National Archives in Tartu (formerly Estonian Historical Archives) used to have a fantastic service level (as a mentioned a couple of weeks ago). After the opening of the new building they have somehow managed to improve further on it. Orders which used to take a couple of weeks to fill have recently been ready in just days and the fee seems to be even lower than it used to be. I have dealt with quite a few Swedish and foreign archives during the last 20 years, both by mail and in person, and Tartu stands without equals thus far.
Ordering scans of entire volumes is obviously a bit hit and miss, but occasionally something interesting turns up. This time I found some of the missing material from 1692 about the Russian Old Believers and their first arrival in eastern Livonia. Documents mentioned by Gustaf Adolf Strömfelt some years later turned up in a volume where I didn't expect them to be.