Gmail is a great tool. More than 100 million people use it, including me. Google (Nasdaq: GOOG) knows this, of course, and in its efforts to grab share from market leaders Yahoo! (Nasdaq: YHOO) and Facebook, the Big G has taken to inserting display ads into Gmail windows.

"We're always trying out new ad formats and placements in Gmail, and we recently started experimenting with image ads on messages with heavy image content," Google said in a statement to Search Engine Land, which broke the news.

As a Google shareholder, I'm interested in taking a closer look at this latest profit play. Unfortunately, I can't. I've been shut out, and it's entirely my own fault.

How so? I'm an advanced Gmail user. I've tricked out my Mac's Chrome browser with a series of plug-ins, including the Gist add-on that dominates the right side of the screen in my Gmail window. Ads that might otherwise go there are overwhelmed by Gist's code.

Gmailandgistscreenshot

Will having a rich market for plug-ins and extensions ruin Google's chances to gain on Yahoo! and Facebook and separate itself from laggard AOL (NYSE: AOL) in display advertising? That's probably going too far. But we also know right-side display ads work.

Take Facebook. Research firm eMarketer says the social superstar is on track to book more than $4 billion in global revenue in 2011. And in August, Chief Operating Officer Sheryl Sandberg let slip that some of the site's biggest advertisers had increased spending 10-fold.

Gmail has yet to prove it has the chops to attract this sort of outsized interest from display advertisers. But with plug-ins getting in the way, it may also never get the chance. Should investors care? Let us know what you think using the comments box below. You can also rate Google in CAPS.

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