So you've decided to do some user testing of your product. Great job -- very Lean of you!

I'm a huge advocate of doing user testing -- even at the earliest level. One of my favorite stories on the subject is of a team that was working on a product that involved people recording data into a mobile app throughout the day. In order to do a bare-minimum proof-of-concept, they gave people small notebooks and a pen, and asked them to write down the data as it happened during the day. It cost them around $10 per person, and they were able to find out that people felt it was worth doing the activity (which is a requirement for everything else to work).

While testing a product hypothesis like this can be done at the very early stages, you'll need more sophisticated tools as you progress. Once you're at the point where you've got an app (web or mobile) product, user testing sites can be cheap and helpful ways to get feedback from people who you might not ordinarily interview. Keep in mind, it's best to try different testing companies if you're doing a large number of tests. Different companies have different ways of assigning people to tests, so the more companies you work with, the more likely you are to avoid any group bias.

If you're just getting started at the mockup stage, Verify is a great starting point to get feedback on mockups and static content. The price of admission is worth the time you'd spend finding people on Craigslist and coordinating your own review of mockups.

The best site to get started with user testing of a web app is, in fact, UserTesting.com. The value of having video and audio is worth the peak price of $39 for at least a few tests. I know one team that revised one big UI interaction based on the first video, because the user did something that was totally reasonable, but was not what they expected.

TryMyUI is another site that has a similar model to UserTesting.com, and is a good starting point. $35 for a single test, so you can decide between TryMyUI and UserTesting.com to see which site you prefer. (Although they're offering the first test free right now).

Once you get a few initial tests done, and you get a feeling for how to get the best information from your test group, you can move on to some heavier services.

Userlytics is aimed at larger groups who want to do several tests. They include video of the user's face, some additional narrative and highlighting which is great if you're looking to dive in a bit deeper, but you aren't ready to bring in a professional UX team yet.

YouEye is similar to Userlytics in that they offer video of the user, but they have a package approach that's better once you start using the site heavily.

Beyond these services, you want to start bringing in experts. Teams like Adaptive Path and EchoUser have seen it all and can take a deeper dive on your product as a whole, instead of the component-at-a-time approach that these self-service sites tend to focus on.