Q: What is a good SSD free space wipe application?

If you are referring to SSDs, then it's probably not a good idea to do that. I believe that's the main reason Secure Delete was removed. Let the garbage system take care of it. Enable TRIM if the SSD supports it.

Storing and erasing data works differently on an SSD than it does on a regular drive. Garbage collection depends on the brand of SSD and the controller they use. Whether or not to enable TRIM depends on whether or not the manufacturer will suggest it or not and whether or not it's compatible with the OS. In most cases I would say to go ahead and enable TRIM. Specify your brand and let others tell you their thoughts. I assure you they will be varied in both direction as to enable it or not.

With both the garbage collection of the Controller in the SSD and TRIM enabled in the OS, it will take care of the drive as needed and you shouldn't worry about performing any other maintenance to it.

For the issue of secure deleting, I can't really explain as to why it is not good for an SSD. But it will degrade it and shorten the life. You can google the subject on erasing an SSD & also using TRIM. There is a ton of articles and discussions on this matter.

If you are worried about security, regular drives can be read with software also. If security is really a great issue, then you are probably better off with a regular drive where you can safely secure delete. But Apple has removed the secure erase from the OS, so you will have to use other software to do that.

If you what to have the data from free space be unrecoverable on an SSD then use FileVault to encrypt the data. Once you do that the free space is encrypted and unrecoverable without the FileVault passkey. When you are getting rid of the SSD just reformat that removes the key from the disk render the data unrecoverable forever even with the key.

Great suggestion Allan. I never think to advise FileVault as I've never found the need for using that level of security, so it never enters my mind.

It is about the only work around available for incapacitation of data on an SSD.

Currently in my Mac Pro I have a DIY Fusion Drive which is made up of a 1 TB disk drive and an Accelsior PCIe SSD card. When I finally get rid of that is my plan as to how I will handle the data on that fusion drive.

Yea, I've seen that in the past and kinda know that, but encryption is just not part of my thinking process. I have no portable computers, so it's just never been an issue for me. But I have to remember that if I ever decide to get rid of my iMac with the SSD.

Secure delete exists purely because of the nature of magnetic storage, such as that found in HDD's. It is a completely moot point on an SSD.

On magnetic disks there can be a residual trace of the original data that the likes of the CIA, FBI, NSA or James Bond could retrieve what the original data was. Secure erase overwrites the bits where the file(s) you've chosen to secure erase multiple times in an effort to destroy that residue. Seven erase passes is the government standard. The highest number of erases (35) is apparently believed to be so secure that even the most advanced hacker won't be able to get the data back. Imagine running this technique on an SSD; you'd wear out the flash chips faster reducing its lifespan.

There is no such residue on a flash storage drive such as an SSD. Once the data is deleted and there is no longer any record of which blocks that data sat on, or those blocks have been reused, there is no way to get that data back.