Our best referrals come from small IT service companies. I want to come up with an incentive for them to refer us to their clients.

I'd say the best source of ideas on more effective referral programs are to ask your referral sources. Even surveying them with a very simple survey could not only provide some added insights but also reinforce your gratitude for their continued loyalty.

What you might be under-estimating is how good the IT Service firms referring you look when they're able to get their clients a great web development outfit on time and on budget, or at least, I presume this to be the case ;) There are so many bad web shops out there, that just having a reliable referral source might be value enough.

Also, if referrals are being made by only a handful of employees inside these IT service firms, you might consider just spending the fee on a great regular dinner or lunch. High-touch relationship building is often more effective as a lubricant to further business amongst service providers than elaborate referral / incentive programs.

Happy to talk to you further about how to evaluate your options.

In my experience incentives for referrals don't work that well. Particularly if your referral sources are small businesses. The small amount they'll get won't compensate the risk of upsetting their clients. I'd suggest you find a way to make it easy for your referrers to introduce you. Think of a service they could offer their clients you could provide for free: website review or a 1/2 hour consultation. Or a product they could give away that's seems valuable like a complimentary webinar, or white paper or report, or checklist. Help your referrers look good in front of their clients.

OK

First of all, what problem are you solving for them? Why are they referring you?
This is really powerful stuff. This is better than a buzz.
If you incentives them, then they are no longer referring you, they are selling you.

Do you really want to loose that?

Seriously, it sounds like you are doing great!

Steve