I know a lot of people love GoToWebinar, but I prefer methods where the client doesn't have to download anything to their computer. That makes it feel like a bit of a dog to our people.

Depending on your budget and needs, here are some that we've used and liked:

Instant Teleseminar:
Pros - Low-cost entry, can embed the code on to your own page, public chat room available, very reliable (I've been using it for five years and have never had any failure or technical issue and we've had up to 3,000 participants on this one.)
Cons: Cannot download the replay in video format, but can replay it and embed. Also, this is a powerpoint only type webinar (no screen share or "you on camera")

Meet.fm:
Pros - low-cost entry point, chat, pop-up boxes, can brand with your info, analytics on exactly who watched your replay, versatile - you on camera, powerpoint, play videos, share your screen, do it all. Stable with crowds (we've had up to 3,000 on this one too.)
Cons - no embedding, a little buggy on the audio (guests sometimes have to use the phone in option.)

WebinarJam:
Pros - One-time investment instead of monthly, nice "wrapper" for Google Hangouts, dynamic - lots of functions and lots of ways to share info.
Cons - Higher learning curve, higher one-time investment

22Social:
Pros - Easy to use (registration page, view page are one), Nice templates to choose from, reliable and sound (no tech issues ever), dynamic (too many options to list including a built in affiliate program.)
Cons - Facebook only (but if this is where you are meeting with your audience or if you are selling a social media product, then this is THE tool.)

There are so many more out there, but I just wanted to share those I have personally used often.

Best of luck with your research and your project!