Ok, so when did you really start listening to MGMT, and when was it you only listened to The Tough Alliance for an entire month?
Last Playlists tries to answer these burning questions, as well as giving you sweet little nostalgic flashbacks. By collecting loads, and loads and loads of statistics from your last.fm account, monkeys and tiny squirrels put together playlists that exactly define what you were listening to, month by month.
Given the fantastic database at last.fm, your hard work with scrobbling all your songs, and the secret sauce of the Last Playlists Algorithms, all of this is possible by simply entering your last.fm username at the homepage.
A few words about the playlists: while the technical details are just too boring (and, it's also the secret sauce, as mentioned above), the playlists favor songs you haven't listened to before the time period in question. Also, to prevent sad faces and boredom, the playlists prefer an artist to appear no more than once, although if you actually listened to The Tough Alliance for an entire month, you will get nothing else on the list. Finally, as a corollary of this, you won't get a playlist at all for a month, if you haven't listened to enough new music for that time period.
Also note, that while Last Playlists create playlists on a monthly basis, last.fm only offers scrobble history on a weekly basis. Needless to say, this will lead to a few days of scrobbles ending up on the wrong month's playlist in the beginning or end of a month. That's just the way life is. Also, this means that last months playlist won't be generated until sometime on during the first Monday of the following month. I.e. January's playlist can't be generated until the first Monday of February.
Finally, if you are of the inpatient kind, you will be annoyed by the time it takes to suck up the scrobbles from last.fm. That's just the way it is. It's fine to leave the page, the next time you return, just type the username again, and the playlists will be ready for you.
Big Fat Disclaimer: Last Playlists is some sort of beta. And not the GMail kind of beta, but in the old sense. You, the server and last.fm can probably botch the system quite easily. Do not rely on this service for matters of life and death. It's prohibited to use Last Playlists for life sustaining treatment.
Also, using a browser that doesn't know about HTML5 will probably render horrible pages. If you want to use old Internet Explorer versions, go ahead, but please don't use this service.