Usability is an important part of any product (if it’s too difficult to use, it  doesn’t matter how great the functionality is) and thereby an important characteristic for the testing team.

But when reading about usability testing, it often involves an outside person trying to use a feature for the first time. Now this is a good thing, but there is a lot more to Usability than Learnability.
There also exist systematic inspection methods, e.g. Heuristic Evaluation by Jacob Nielsen, that should be performed by super-experts on usability.
He recommends alternating between user testing and evaluation, but I think a manual software tester can do both at the same time, if she has (some) knowledge about the domain, (some) knowledge about the product, and (some) knowledge about usability problems.

Here is a very cheap way to do usability testing; ask the following questions to someone with many hours of using the product (probably a real user or a manual system tester)

General usability: What’s your most important usability issues?
Affordance: Would you say that the products invites to discovering possibilities, or are there features you learned too late?
Intuitiveness: Is it easy to understand and explain what the product can do?
Minimalism:Have you seen redundant information anywhere? Any important information missing?
Learnability: Are there any areas you feel are difficult to understand and learn?
Memorability: Do you forget how to perform some actions?
Discoverability: Can you discover all available operations by (systematic) exploration of the user interface?
Operability: Are any common operations cumbersome?
Interactivity: Are there situations where you aren’t sure of which kinds of interactivity is supported?
Errors: Have you encountered strange error messages, or situations that were difficult to recover from?
Consistency: Is the user interface annoyingly different anywhere, or hard to understand?
Tailorability: Are there any (default) settings you would like to change?
Accessibility: Did you notice any problems while using High DPI and mostly the keyboard? Color-blindness?
Localization: Any experiences of running translated product or data?
Documentation: Are there any information you would like to see in the Help?

You might already know about many of the issues in the answers, but now you know more about their importance.
You won’t get any quantitative measurements, but hopefully a lot of valuable information.

Even better might be to have questions like these in the back of your head when executing manual tests, you’ll probably get valuable information for free. There is no need to memorize the categories, the important ones will emerge.

Now and then I read that testers don’t know enough about Usability, that there is a need for a Performance Testing expert, that a Security consultant should be called in, or that a master of the used technology would make Installation and Compatibility testing possible.
This might be true in the general case, but there are many testers that are working with the same product suite for years, and over time you become an expert of most aspects of quality for your specific product.

Let’s look at some examples:

One sub-category of Usability is Operability; that an experienced user can perform functions in a fast and efficient way. As a system tester, you have done most operations many times, and you know what you can expect from features in the sense of operability. You can point at places where for instance the ability to delete multiple items with a few clicks would make a difference.
Learnability is another sub-category of Usability. You only learn the basics once, but maybe you have heard customer stories of confusing things, or you could let a new member of the test team think in this direction.
Regarding Accessibility it doesn’t cost too much to use High DPI, speakers turned on, Code Blind or Magnifier sometimes.

Large-scale Performance might require an expensive tool, but I bet you’re doing some simulations without them; maybe just by telling the whole development team to hit the same server at the same time.
There is also a small-scale, low-level Performance aspect that shouldn’t be understimated. If a dialog takes more than a second to display, it might be something that put the user’s confidence in doubt.
If you have tested your product for quite some time, you will immediately notice when something takes just a bit longer than it could take. There of course might be valid reasons for this, but talking to the developer about it might be beneficial to both the producers and consumers of your software.

Security testing seems more difficult, but as a product tester you know at which moments authentication takes place, you know if there are passwords stored, and that they should be encryted; you might not know how to exploit a crash, but you are an expert at provoking the crashes.

For Hardware/OS/Application Compatibility testing you will become more of an expert the longer you work with them, at least if you have the curiousity to learn more things when you get the chance.
You might not know a lot about how the iPhone works, but you know all the details that can be used for interacting with your web site.

Sometimes I also read the extreme that testers shouldn’t bother with Usability/Security/Performance testing, which to me seems like an incredible waste of knowledge and resources.
When testing functionality manually, you can look at quality attributes at the same time, and get a lot of coverage for free.

I’m not saying true specialists aren’t needed, but I’m saying that there are a lot of expertise in your building that at least can be used complementary.
If you are in a situation where you know a lot of these things, but aren’t allowed/encouraged to test these things, I think you should try to convince your managers of a better and more fun way to test your product.