I am overwhelmed with many mailboxes from many of my mail accounts. I use to keep each (regular) contact in the separate mailbox, so it's easier for me to find message, when I need them. (I don't need them often though.)
I do have like 5 or 6 regular contacts whom I receiving a lot of messages from (not less then 3-4 from each one daily). Unfortunately, these guys' names are so different, so every time when I'm receiving messages and want to save them, I need to scroll the whole (left-side) list from the top to bottom to find the right person.
I'm getting bothered with that too often now. So, after a quick look around, I found two plugins that can solve my problem. The first one, which looks cooler, is Mail Act-On 2, however, you have to buy it for $24.95 (or you can still get Mail Act-On 1.3.2 for free). The other one is MsgFiler, which is shareware - you can have it for free, but better pay $8 if you find it useful.
So, starting with the MsgFiler (yes, I'm cheapo :). It comes with installer, so you basically don't have to do anything for it. After that, you need to use global keyboard shortcut configure, to setup shortcut for it.
It's pretty easy and described on the site. Make sure you don't misspell Move with MsgFiler (as I did), because you'll not be able to access it then.
Start up Mail.app, select messages, click Command-9 and see the MsgFiler window. You can start typing the name of mailbox, it will react right away and filter the mailboxes by name.
Well, it's easier then to scroll the whole list, but still something could be made better.
Okay, so lets try the other one, which is also free. It also comes with pretty easy installation. But the principle of Mail Act-On is very different. What you do, is to create a rule for every action that you want to have. Like I did, for messages from my brother, linked to o.
That was the first thing that I didn't find very comfortable. Actually, it is comfortable for some cases, but pretty limited in other.
Everything else looks pretty easy though. And much more fancy than MsgFiler.
After spending half an hour with each one, I decided to stick with MsgFiler as for now. It gives me better flexibility, as I don't want to assign a lot of contacts to the key. I know that I'll end up with map of 10 or 15 keys, and it's going to become one more big list to scroll through. I'll miss the fancy look though. ;)
Quick update: As it works out, MsgFiler starts to complain about being unregistered after the 5th message. While $8 is not a lot, I find it to be annoying. I am happy to pay for software, but 5 messages isn't enough for me to find out do I really like it and want it. It'd take a week or even more to finally settle up. (Sorry that I am so slow.) So I'm turning into Mail Act-On back.