This is a big release not only because it has a new game mode, but because it brings in a lot of changes which may have been lost over the last two years, and makes further changes a breeze to make in comparison (if you want to know more about this, I explain it briefly Technical (please start the thread subject with [osu!mania]).
Apart from osu!mania, there are a lot of other changes, all of which I cannot list in this one place. But here are some that are important to note:

osu!mania notes:
On a personal note, this is the largest single-chunk feature another dev has contributed to osu!. I hope going forward we will see more of this, as it really helps keep the ball rolling while I am busy fixing bugs and keeping things running smoothly. I can't promise anything, but there may be another game mode in development in the not-so-distant future, so stay tuned
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osu! on.
Apart from osu!mania, there are a lot of other changes, all of which I cannot list in this one place. But here are some that are important to note:
- Beatmap processing is now many times more efficient.
- BSSv2 is now live. The first time you update existing maps will do a full submit, but it will be super fast (and simple!) after that.
- Per-object hitsounds are in the editor (kudosu! to mm201 for this one). No more 10000+ timing sections to get your map sounding right. (we've been using this on osu!stream for a while now and you can't go back.) You can quickly set these using Shift+QWER and Ctrl+QWER.
- Allow recovery from failing in multiplayer by reaching full HP.
- Scrolling to in-game online ranking list.
- Fixes to skin select to make it play more realistically.
- If your beatmap database corrupt or got processed wrong, include osu!.db.xxxxxx.bak in your post (attach using puush or mediafire).
- If your scores database corrupt or got processed wrong, include osu!scores.db.xxxxxx.bak in your post (attach using puush or mediafire).
- If you are seeing a graphical issue, please include a screenshot or video.
osu!mania notes:
- Please discuss this mode using the #osumania chat channel.
- Online ranking is disabled until has been received and scoring and conversion is finalised.
- Please don't submit osu!mania maps yet.
- osu!mania editor is still very much a WIP.
- A lot of +difficulty mods are not yet finished/finalised.
- An updated template skin will be released soon. You can find more details on skinning at the wiki page.
- Adjust speed of notes with Ctrl +/-
On a personal note, this is the largest single-chunk feature another dev has contributed to osu!. I hope going forward we will see more of this, as it really helps keep the ball rolling while I am busy fixing bugs and keeping things running smoothly. I can't promise anything, but there may be another game mode in development in the not-so-distant future, so stay tuned

osu! on.