Not officially available yet / Not available.
With Android 5.0 Lollipop's release, the recents view (multitask button) became overview, and instead of showing you only actually running apps, it also shows you apps that have ran in the past. Over time, this screen ends up listing pretty much every app you have ever run, becomes completely useless, and wastes a perfectly good (soft-)button.
Recently aims to restore sanity, by limiting which apps are shown. In its aggressive 'running tasks only' configuration, it will show you only apps that are actually running right now. Because Android can be fairly aggressive itself with killing off apps, apps you have used just a minute ago may disappear from the list in this setup, and configuring Recently to remove entries that have not actually run the past few days may be the better option.
Aside from listing only running apps or apps that have run in a certain timeframe, Recently also allows you to limit by number of entries shown. Note that actually running apps will always be shown, regardless of this setting - only entries from apps that aren't actually running are ever removed.
A number of more advanced options are available as well, such as showing a 'Clear All' button in the recents view (doesn't work on all devices), limiting the number of Google Searches that show up (originating from the Google search bar and Google Now), and hiding the Settings app.
Additionally, you can force remove tasks and documents that are running but have not been accessed longer than the configured age limit. This is a rather extreme measure that will result in tasks being killed, do not use it lightly.
Purchasing the Pro version supports my developments and will allow you to start Recently automatically at boot.
This app is built specifically for Android 5.0 and 5.1. Future versions may work differently, the recents/overview screen may be fixed, etc. In a year this app may no longer be relevant.
Aren't task killers bad?
No tasks are killed by Recently by default - it only removes entries from the recents list that are not actually running at time of removal. Some of the advanced options can cause tasks to be killed however, but these are clearly marked. Task killing can ultimately be bad for performance and battery life, so be sure you know what you're doing and why.
What resources does this use?
Virtually none. There are no wakelocks at all. Battery impact should be less than 1% worst case.
(1) Make sure you have enabled the switch in the top right of the settings screen
(2) Make sure you aren't using a custom kernel that disables logging
(3) Make sure you give it a minute or so after first enabling it
When I press the back and the multitasking button really quickly, my last used app that is now closed is still listed
The delay in this case is about a second after the app close animation finishes
I just pressed the back button on an app, and I'm in 'running apps only' mode, but the app is still listed
Not all apps actually close when you press the back button. Most do, but some don't.
Nothing is happening at all, and I'm finding an 'unexpected EOF' error in logcat
Sometimes the app can't access logs. This is usually because multiple apps are trying to read from logcat at the same time. The app will keep trying periodically. Rebooting usually gets rid of this problem.
Discussion / support
The discussion and support thread can be found here: http://forum.xda-developers.com/showthread.php?t=3050998