The first thing to say, is that I want to be honest and I'm affiliate to the software I present bellow (I'm the software designer/programmer of this font identification engine) and I'm really proud about it :)
It's not an online service but a software application that runs on your Mac or Windows PC:
It takes as input a Text image and identifies the font of any letters you select, by examining both your local fonts (installed or stored in a folder) and also querying the Find my Font online font database (currently about 125.000 fonts).
The reason I decided to post a message here, is that Find my Font has many advantages over the traditional WhatTheFont and WhatFontIs services, especially considering Google Web Fonts matching.
Nothing is actually free on free services, everything has it's price when it comes to performance or final results.
WhatTheFont identifies only the fonts they sell (and most of the time they can't even accurately identify their own fonts). There is no reason ever to add Google Web Fonts on their database. In fact I think they wish that Google Fonts never existed.
WhatFontIs has a lot of free & commercial fonts in their database, but it's not their primary concern to add & update free fonts, as their main income comes from affiliation sales of commercial fonts from MyFonts.com (the owner of WhatTheFont service), fonts.com and other commercial font resellers. More than this, it will not tell you that the free font they identified belong to Google Fonts.
FindMyFont is not a "free service". You can download the 30-days-trial but if you really like it you have to buy the Pro edition (once).
What you get in exchange for this (non-free service) is that we have no affiliation ties, so we can focus to offer you the best service:
a) The current online database of Find my Font contains all Google Fonts published until now. They are stored in a special group and the App always let you know that the font you identified belong to Google Fonts.
b) The same thing applies to other free/fremium/commercial fonts sources as our online database contains the complete collection of Dafont.com, OPTI Fonts, etc. as well as more than 300 commercial foundries.
c) If you want to find a Google Web Font which is a good substitute of a commercial font, there is no reason to create any images: Find my Font has a special function to select a font of your computer, type the letters you want and find all similar fonts (including Google Fonts).
d) If you want to focus your results only on Google Fonts, you can download the whole Google Fonts set on your computer, let Find my Font know where these fonts are, and find an equivalent / similar font within seconds (the local font identification speed is about 5 million fonts / minute).
More than that, the matching algorithm of Find my Font is superior in many ways to WhatTheFont and WhatFontIs: You can use images with text as small as 20 pixels high, it accurately matches both the font-family and the weight, it can match samples artificially italicized / emboldened / expanded / condensed and many more.
I declared off-course that all above info comes from a person affiliate to (and very proud of) the software presented here.
Too good to be true?
You can always try it yourself by downloading the 30-days-free trial:
You will love it!
UPDATE (31/3/2015) Satisfying the customer requests we received, the new Find my Font version 3.3.02 now supports the following options for Online Font Matching: "All", "Freemium", "Commercial" and "Google Web Fonts".
Google Fonts will still appear in results if you select "All" or "Freemium", but if you select "Google Web Fonts" the matching results will contain exclusively Google Fonts + a matching percent for each one of them.
This makes it even easier to choose the closest Google font to your image sample or to any font you have on your computer.