Google sometimes announces usability studies (e.g. for Blogger in November 2006). Pete from Team Blogger has posted about another opportunity to play with Blogger for an hour or so and make up to $100 for giving Google your feedback - which might even be done online or in person outside the US, so non-Californians can apply if they want to!
You can view their FAQ and sign up to register your interest, which involves filling in a questionnaire, probably takes about 10-15 minutes. There's no guarantee they'll invite you to take part, however. I signed up ages ago and have never heard a peep from them, but c'est la vie.