user11648: Since I updated to El Capitan Public Beta, my Arduino Uno doesn't show up in the serial devices list.
The System Information utility shows an Arduino connected on the USB port (as 'Communication Device'), but a serial port does not get created at /dev. 
The board works on Windows and the previous versions of El Capitan.
The problem persists on beta 6.

user13861: I have the OS X 10.11 installed and was unable to upload to my Arduino board. I always got stuck with the error code
avrdude stk500_getsync() not in sync resp=0x00

After installing the FTDI driver it didnt't work either. So I figured out Apple provides it's own FTDI driver kernel extension since OS X 10.9, named AppleUSBFTDI.kext. So the AppleUSBFTDI kernel extension interferes with the FTDI driver. An USB port shows up in the Arduino Application, but no communication involving avrdude is possible. 
Solution (without modifying or disabling any system security options introduced in 10.11):

Open a terminal
Execute: sudo kextunload /System/Library/Extensions/AppleUSBFTDI.kext/ -> this will unload the Apple FTDI Kernel extension.
Reinstall the FTDI driver
Start the Arduino IDE, select a Com Port, the correct Arduino board
Compile and upload your sketch to the board.

Please note that kextunload will only temporarily unload the module. When your system gets rebooted, you have to do this step again. It's possible to remove the kernel extension, but out of security reasons I don't describe how to do it in this post. If you're familiar with the console, you know how to do this :)

user11086: As noted elsewhere, there have been plenty of problems with the CH340 driver and the OSX El Capitan betas, culminating into the driver flat out not working in the latest beta. This is either a bug with OSX or a bug in the driver. I would lean towards this being a bug in the driver, which hasn't been updated since late 2013.
There are other 3rd party drivers that might work on El Capitan. mac-usb-serial.com is the most reliable looking one that I could find, and it does appear to be actively developed. It does cost money, but it is signed (which the mfg. driver isn't) and it has a test utility you can use before purchasing.
The only other options after that are:

Hope that WCH update their driver (don't hold your breath).
Program using an external programmer
Buy an Arduino not using the CH340


11648: <p>Since I updated to El Capitan Public Beta, my Arduino Uno doesn't show up in the serial devices list.</p>

<p>The System Information utility shows an Arduino connected on the USB port (as 'Communication Device'), but a serial port does not get created at <code>/dev</code>. </p>

<p>The board works on Windows and the previous versions of El Capitan.</p>

<p>The problem persists on beta 6.</p>
 13861: <p>I have the OS X 10.11 installed and was unable to upload to my Arduino board. I always got stuck with the error code</p>

<pre><code>avrdude stk500_getsync() not in sync resp=0x00
</code></pre>

<p>After installing the <a href="http://www.ftdichip.com/Drivers/VCP.htm" rel="nofollow">FTDI driver</a> it didnt't work either. So I figured out Apple provides it's own FTDI driver kernel extension since OS X 10.9, named <em>AppleUSBFTDI.kext</em>. So the AppleUSBFTDI kernel extension interferes with the FTDI driver. An USB port shows up in the Arduino Application, but no communication involving avrdude is possible. </p>

<p>Solution (without modifying or disabling any system security options introduced in 10.11):</p>

<ol>
<li>Open a terminal</li>
<li>Execute: <em>sudo kextunload /System/Library/Extensions/AppleUSBFTDI.kext/</em> -> this will unload the Apple FTDI Kernel extension.</li>
<li>Reinstall the <a href="http://www.ftdichip.com/Drivers/VCP.htm" rel="nofollow">FTDI driver</a></li>
<li>Start the Arduino IDE, select a Com Port, the correct Arduino board</li>
<li>Compile and upload your sketch to the board.</li>
</ol>

<p>Please note that <em>kextunload</em> will only temporarily unload the module. When your system gets rebooted, you have to do this step again. It's possible to remove the kernel extension, but out of security reasons I don't describe how to do it in this post. If you're familiar with the console, you know how to do this :)</p>
 11086: <p>As noted <a href="https://arduino.stackexchange.com/questions/13998/arduino-nano-with-ch340-reboots-on-os-x-el-capitan/14005#14005">elsewhere</a>, there have been plenty of problems with the CH340 driver and the OSX El Capitan betas, culminating into the driver flat out not working in the latest beta. This is either a bug with OSX or a bug in the driver. I would lean towards this being a bug in the driver, which hasn't been updated since late 2013.</p>

<p>There are other 3rd party drivers that might work on El Capitan. <a href="https://www.mac-usb-serial.com/" rel="nofollow noreferrer">mac-usb-serial.com</a> is the most reliable looking one that I could find, and it does appear to be <a href="https://twitter.com/osxpl2303" rel="nofollow noreferrer">actively developed</a>. It does cost money, but it is signed (which the mfg. driver isn't) and it has a test utility you can use before purchasing.</p>

<p>The only other options after that are:</p>

<ul>
<li>Hope that WCH update their driver (don't hold your breath).</li>
<li>Program using an external programmer</li>
<li>Buy an Arduino not using the CH340</li>
</ul>
