5 Ways to Get Usability Testing on the Cheap

I think this article has some good tactics. Three additional points I would add:

1. An incredibly valuable skill is being able to clear your mind and see what you're working on from the fresh perspective of a potential user. This is even easier if you are your own target audience, or if your product is such that your team can 'eat your own dogfood.'

2. Don't forget that usability testing typically only makes sense if the participant is in your target audience. If you are targeting some sort of specific niche audience or need (and most businesses probably are), it's worth taking time/$ to find the right kind of person versus someone online who has no context for the need you're trying to solve. If you already have customers, some of them will almost certainly be excited to help test new features.

3. If you're doing research interactively, consider having someone who didn't design the product asking the questions: it's really easy to bias the results based on your expectations.

Finally, it's certainly not cheap but if you ever have a chance to do usability research in a lab with genuine 1-way glass, it's a blast!